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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?

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)

WHAT IS A PLASMON POLARITON?

[1] W.L. Barnes, A.Dereux, T.W. Ebbesen, Nature 424 (2003) 824-830


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


•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


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.


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.


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.


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.


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 )


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


Raman Characterization

All peaks are red shifted with increasing temp.Decreasing stress with temperature increase2D peaks narrow with increasing temperature


Si faceC face

What would a H2 etch do?


Experiment:

Mathematical Model [7]

Surface Plasmon Polariton (SPP) in Epitaxial Graphene

2

2 2

int 2 2

(1 )( )[ ( ) ( )]

2er F F


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.


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

17


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


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


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


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


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


RPA approximation:


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!


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

34

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

38

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

39

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

40

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?

41

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

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plasmon polariton

viability of ir plasmonics

epitaxial graphene

electromagnetic field

electromagnetic waveelectric

manchester electrons

nearfield phenomenon

spp surface plasmons