Post on 20-Jan-2015
description
Clean Energy Lab (CEL)
Towards Plasmonics in Epitaxial GrapheneM.V.S. Chandrashekhar
Department of Electrical and Computer Engineering, University of South Carolina
1
USCG.KoleyT.S. SudarshanC. WilliamsJ. WeidnerB.K. DaasK.M. DanielsS. ShetuO. SabihA. Obe
CMUR. FeenstraN. Srivastava
MPI/PisaU. StarkeC. Colletti
OUTLINEClean Energy Lab (CEL) @ USC
•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 2005-Physics 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 surfacesensitive to surroundings Very transparent and highly conductive-touch screens?
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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)
WHAT IS A PLASMON POLARITON?
[1] W.L. Barnes, A.Dereux, T.W. Ebbesen, Nature 424 (2003) 824-830
Clean Energy Lab (CEL) @ USC
1. Overcome diffraction limit of light (d<λ/2) using SPP2. Merge electronics and optics together in nano scaled range3. Important for data processing, super lensing, sensing etc.
MOTIVATION: THE PLASMONIC CHIP
5
CHALLENGE: Couple Collective SPP to Single particle excitationsSPP
[2] M. Dragoman, D. Dragoman, Nanoelectronics: Principles and Devices, Artech House, Boston, 2006
Surface Plasmon Polariton at metal/dielectric interface
2
2( ) 1 p
m
When <0, K is imaginarySurface confinement
m
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•SPP propagation mediated by intra band processes•SPP detection mediated by inter band processes
Graphene
2
2 2
int 2 2
(1 )( )[ ( ) ( )]
2er F F
e i Ei dE f E E f E EE i
2
2
int
( )]F
ra
ef E E
i dE Ei E
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
HOW DO PLASMONICS WORK?
SIC SUBSTRATE DIELECTRIC FUNCTION
Clean Energy Lab (CEL) @ USC
2 21
2 22
( ) LOSiC SiC
TO
i
i
WLO= Longitudinal optical phonon (972cm-1)WTO= Transversal optical phonon (796cm-1)At high frequency ~6.5 [8]At low frequency ~9.52
SiCSiC
[8] Dmitriy Korobkin, Yaroslav Urzhumov, and Gennady Shvets; J. Opt. Soc. Am. B, 23,3,468 (2006)
LST relation:2
2
(0)
( )L
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.
Clean Energy Lab (CEL) @ USC
22
22
20
1[1 ]
( , )( )
qqcc
Viability of Plasmonics in EG on SiC
TM modes are found by assuming that the electric field has the form as..
1iqz Q xxE Be When x>0
When x<0
and 1iqz Q xzE Ae 0yE
2iqz Q xxE De and 2iqz Q x
zE Ce 0yE
1 2
2 202 21 2
2 2
( , )q i
q qc c
Dispersion relation for TM mode is given by
Assuming we are in low q, so q<w/c, SPP dispersion relation is.
Fig: SPP dispersion relation plot with free space dispersion
450
8
Free space dispersion relation is qc
SPP dispersion intersects the free space dispersion -coupling of SPP into free space radiation- SiC substrate
essential.
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Viability of Plasmonics in Epitaxial GrapheneCoupling between SPP and Single Particle Excitations
Applying single particle excitation boundary condition for intra and inter band scatteringComes from graphene E-k bands (developed by S.Das Sarma)
1 Fv q
2 0 2 Fq k2 2 Fq E 2 Fq k
q= wave vector = frequency
9
•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.
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MODULATING EPITAXIAL GRAPHENEPLASMON 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.
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Graphene
Exfoliated graphene( single layer)
Epitaxial graphene (single or multi layer)
Silicon (Si) GaAs 4H-SiC Metal (Ag)
Graphene
Supporting TE mode
--- --- ---- No Yes [2]
Dispersion relation
Parabolic parabolic parabolic parabolic linear –EHP at any wavelength
Band gap 1.12eV 1.42eV 3.23eV 0 0 Electron Mobility (cm2/v-s)
<1400 <8500 <900 200000
RMS roughness --- ---- ------- ~1nm <0.5nm
SPP Detection and guiding materials
----- ------ -------- Metal to guide, Semi to detect
Single material for guiding and detection, 11
[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 )
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A B C
A
B
B
A
A
C
C
6H-SiC Graphene
FiG: Realization of Graphene from 6H-SiC
12
Epitaxial Graphene GrowthRaman XPS & ARPES
D peak (1345 cm-1)…..due to induced disorder
G peak (1585cm-1)… due to in plane vibration
2D peak (2670cm-1)…..due to double resonant process
ID/IG…Disorder ratio <0.2 [5]
[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
5µm× 5µm
Growth mechanism is step flow mediated [*]
Growth mechanism is defect&step mediated [**]
What happens
in between?
[*] M. Hupalo, E. Conrad, M. C. Tringides http://arxiv.org/abs/0809.3619 [**] Appl. Phys. Lett. 96, 222103 (2010)
13000C 13500C 14000C 14500C
Si face
C face
A plane
M plane
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Raman Characterization
All peaks are red shifted with increasing temp.Decreasing stress with temperature increase2D peaks narrow with increasing temperature
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Si faceC face
What would a H2 etch do?
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Experiment:
Mathematical Model [7]
Surface Plasmon Polariton (SPP) in Epitaxial Graphene
2
2 2
int 2 2
(1 )( )[ ( ) ( )]
2er F F
e i Ei dE f E E f E EE i
2
2
1 ( ) cos( 1)1 2 0 / 1 0
1 ( ) cos( 1)1 2 0 / 1 0
Nc
RN
c
211 [( sin 1)]
2cos 1
nn
2
2
int
( )]F
ra
ef E E
i dE Ei E
Our approach
Fig: Schematic view of FTIR differential reflection spectra setup
16
2 21
2 22
2 2( ) LO
TO
i
i
[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)
Blank SiC is used as reference.
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Variable ParameterNumber of Layer, NFermi Energy Ef
Scattering time τ
Surface Plasmon Polariton (SPP) in Epitaxial Graphene….(Cont.)Results of developed mathematical model
2
2
1 ( ) cos( 1)1 2 0 / 1 0
1 ( ) cos( 1)1 2 0 / 1 0
Nc
RN
c
Fig: Variation of number of layer Fig: Variation of Fermi level
Fig: Variation of scattering time
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Clean Energy Lab (CEL) @ USC
Experimental results from FTIR: Evidence of SPP at EG/SiC interface
Fig: AFM image of SiC Substrate
Fig: AFM image of EG (2ML)on SiC
Fig: IR reflection of SiC Substrate with SiC as reference
Fig: IR reflection of EG with SiC as reference
Surface Plasmon Polariton (SPP) in EG/SiC interface
LO
TO
18
Clean Energy Lab (CEL) @ USC
Extracted Parameters:1.No of Layer N=2-172.Fermi Energy Ef=10535meV3.Scattering time, τ=4-17fs
Interband broadening is assumed constant=10meV i.e. only intraband scattering considered.
EG transport properties extraction using FTIR
Extracted No of layer matches well with XPS measurements.
Fig: IR reflection measurement and mathematical model are consistent
Clean Energy Lab (CEL) @ USC
0
( ) ( )s Fn D E f E E dE
2( ) 2 / ( )FD E E v
11( ) / F
s
k vn
EG transport properties extraction using FTIR
Carrier density
Fig: Fermi level Vs No of layer
Fig: Scattering time Vs avg. carrier density
Fitting value of k1=0.6 suggests our EG isdominated by short-range scattering.
Short range scattering[9]
Coulomb scattering[9]
1
sn
sn
Mobility, µ= 2 /F Fe v E20
Mobility (1000-10,000) cm2/V-s
[9] L A Falkovsky “Optical properties of graphene” . Phys.: Conf. Ser., Volume 129, Number 1 (2008)
B,K. Daas…MVS et al JAP (2012)
CORRELATION WITH ULTRAFAST SPECTROSCOPY OF EPITAXIAL GRAPHENE
85fs, ~10nJ 785nm laser, pump &probe Measures ENERGY relaxation time, not momentum τenergy>>τmomentum, supports short range scattering
If states are occupied by pump, probe signal will not be absorbed, transmission increases
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
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SiC Substrate
Graphene
Graphene
Incoming light
source
Collecting light
signal
Mirror
SPP
Sensing element
Fig: Experimental setup
1.Pure N2 - inert gas2.15ppm NO2 -electron accepting gas3.500ppmNH3 -electron donating gas
Findings:Reflection amplitude changes-Looks like change of thickness but thickness can’t change
MOLECULAR DOPING OF EG-LONG RANGE?
23
2
04sSiC F
er
v
22 2
20
sin( )
8 (sin )2
xG x d
x
22 2
20
(1 cos )( )
8 (sin )2
xF x d
x
Clean Energy Lab (CEL) @ USC
Conductivity Matching:Optical Conductivity:
2
0
[4 / (2 )][ ]
[4 / (2 )] 4RPA s i s sT
i s s s
n n F r re
h nG r r n
Intraband-low f Interband high fFig: Dielectric function of SiC
Extracted parameter ni
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
Interband scattering matters even at DC.
2
2
int
( )]F
ra
ef E E
i dE Ei E
2
2 2
int 2 2
(1 )( )[ ( ) ( )]
2er F F
e i Ei dE f E E f E EE i
RPA approximation:
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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 moleculeAgrees with Kelvin probe measurements
Clean Energy Lab (CEL) @ USCNo of Layer
Gas Fermi level(meV)
ni/ML(cm-2)
Intra band scattering time (fs)
Avg. Inter band scattering time(fs)
34 N2 25 2x1011 90-280 185 27-60
NH3 30 6x1012 60-90 75 1.6-2
NO2 35 2x1013 2-9 5 0.3-0.522 N2 45 3x1011 10-17 14 9-17
NH3 65 7.5x1012 2-9 5.5 0.2-2
NO2 95 6x1013 0.9 0.9 0.1-0.29 N2 70 5.1x1011 10-20 15 3-4
NH3 90 5.5x1013 0.8-1 0.9 0.2-0.5NO2 120 1.5x1014 0.4-0.5 0.45 0.1-0.3
CORRELATION WITH ‘DC’ MEASUREMENTS
NO2 makes the C-face more p-type Implied δp~1012-13cm-2 -is this possible?
4ppm
M. Qazi….MVS, Koley et al., Appl. Phys. Exp., 3, 075101 (2010)
CORRELATION WITH KELVIN PROBE
Consistent with F.Schedin’s result of G/SiO2
Assume ΔEf~10meV for 4ppm. μchem ill-defined.
~60% or more change in conductivity expectedScattering from impurities not enough to explainmeasured change in optical conductivity
Electron affinity of NO2 dominates!
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No of Layer
Gas Fermi level(meV)
ni/ML(cm-2)
Intra band scattering time (fs)
Avg. Inter band scattering time(fs)
34 N2 25 2x1011 90-280 185 27-60
NH3 30 6x1012 60-90 75 1.6-2
NO2 35 2x1013 2-9 5 0.3-0.522 N2 45 3x1011 10-17 14 9-17
NH3 65 7.5x1012 2-9 5.5 0.2-2
NO2 95 6x1013 0.9 0.9 0.1-0.29 N2 70 5.1x1011 100-200 150 3-4
NH3 90 5.5x1013 0.8-1 0.9 0.2-0.5NO2 120 1.5x1014 0.4-0.5 0.45 0.1-0.3
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!
From FTIR
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
H+ attracted to graphene cathode 1V, 1hr. Can it react? V<1.2V, H2 formation potential Goal: Bandgap in diamond-like graphanes.
RMS: 0.57nm
RMS: 1.00nm
Scale: 8nm
Scale: 8nm
Before
After
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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
• R. Sharma, et. al. Anomalously Large Reactivity of Single Graphene Layers and Edges toward Electron Transfer Chemistries, Nano Letters 10, 398-405 (2010)
0
200
400
600
800
1000
1200
1200 1600 2000 2400 2800
Ram
an In
tens
ity (
arb.
uni
ts)
Wavenumber (cm -1)
New peak at ~2930
Indicative of C-Hbond
DG
2D
Graphene
Graphane
D-peak red-shifts 1354-1335 cm-1.
G peak broadens and
slightly blue shifts ~3 cm-1
H-FUNCTIONALIZATION SHOWN BY RAMAN SLOPE
•B. Marchon, et.al. Photoluminescence and Raman Spectroscopy in Hydrogenated Carbon Films. IEEE Transactions on Magnetics, Vol. 33, NO. 5, Sept. 1997.
Increasing photoluminescence background Increasing hydrogen content
Ratio between slope m of the linear background and the intensity of the G peakm/I(G) Measure of the bonded H content
Based on amourphous carbon results maybe dominated by grain
boundaries
D peak
G peak
Ra
ma
n I
nte
nsi
ty
Wavenumber (cm-1)
S≈ 18µm
Florescence is not seen in carbon only hydrocarbons!!!
36
FLUORESCENCE BACKGROUND TO ESTIMATE H-CONTENT
Damage distinguished from functionalization by a) damage has unmesurable slope for a given D/G ratio b) D peak position
37
SUBSTRATE DEPENDENCE OF FUNCTIONALIZATION
Substrate D-peakPosition
Before (cm-
1)
D-peakPosition
After (cm-
1)
D/G Ratio
Before
D/GRatioAfter
NormalizedSlope
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
Table 1: Average Parameters From Each Substrate in Study
• Substrate Limited Functionalization– Possible Causes
• Off-cut angle• Substrate Resistivity• Residual Damage in Graphene
Problem: Issue with conversion control? Solution: Enhance reactivity with metal?
* All substrate averages contain at least three samples
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RAMAN SPECTRA OF FUNCTIONALIZATION WITH AND WITHOUT PT NANOPARTICLES
Chemically Deposited Platinum H2PtCl6 · 6H2O + DI water
• Raman Shows:– Incredibly large D/G ratio~4.5– Emergence of Fluorescence– Addition to D’ shoulder peak– C-H peak at ~2930
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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)
SUMMARY: METAL CATALYSIS
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D Position Before (cm-1)
D Position After (cm-1)
ID/IG Ratio
Before
ID/IG Ratio After
Normalized Slope
Before (µm)
Normalized Slope
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 catalystIncrease in fluorescence bandgap?
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SCANNING TUNNELING SPECTROSCOPY
Evidence of localized states
More evidence required to distinguish from damageWhat are these states?
K.M. Daniels, …MVS, R. Feenstra… et.al, presented at EMC2011accepted, JAP
*8x8mm
functionalized
unfunctionalized
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