Without SiC, such an intersection is not possible.
Magnitude of conductivity or real part? When normalizing to SiC Substrate, what is the expression. Is there an angle?
4-nitrobenzene diazonium tetrafluoroborate
Fluorescence does not show in carbon but does show in hydrocarbons
Averages of at least three samples
Would like to correlate slope with
Science Cafe Discovers a New Form of Alternative Energy
Clean Energy Lab (CEL) Towards Plasmonics in Epitaxial Graphene M.V.S. Chandrashekhar Department of Electrical and Computer Engineering, University of South Carolina USC CMU MPI/Pisa G.Koley R. Feenstra U. Starke T.S. Sudarshan N. Srivastava C. Colletti C. Williams J. Weidner 1 B.K. Daas K.M. Daniels S. Shetu O. Sabih A. Obe
Clean Energy Lab (CEL) @ USCOUTLINE•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) @ USCWHAT IS A PLASMON POLARITON? Polariton: Collective oscillation of electrons (Plasmon), generated by the electromagnetic field that excites the metal/dielectric interface . 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)  W.L. Barnes, A.Dereux, T.W. Ebbesen, Nature 424 (2003) 824-830
Clean Energy Lab (CEL) @ USCMOTIVATION: THE PLASMONIC CHIP 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. ωp2 ε m (ω ) = 1 − 2 Surface Plasmon Polariton at metal/dielectric interface ω Whenε m<0, K is imaginary Surface confinement 5 SPP CHALLENGE: Couple Collective SPP to Single particle excitations  M. Dragoman, D. Dragoman, Nanoelectronics: Principles and Devices, Artech House, Boston, 2006
Clean Energy Lab (CEL) @ USCHOW DO PLASMONICS WORK? •SPP propagation mediated by intra band processes •SPP detection mediated by inter band processes Graphene e2 π h2 ∞ ∂f ( E − EF ) σ int ra ( ω ) = i ∫ dE ]E i −∞ ∂E ω+ τ ∆2 (1 + 2 ) e2 (ω + iΓ ) ∞ E σ int er ( ω ) = i ∫∆ dE ( 2E ) 2 − ( hω + iΓ ) 2 [ f ( E − EF ) − f (− E − EF )] π 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) @ USCSIC SUBSTRATE DIELECTRIC FUNCTION ω 2 − ωLO + iΓ1ω 2ε SiC = ε SiC (ω ) = ε ∞ 2 ω − ωTO + iΓ 2ω 2 WLO= Longitudinal optical phonon (972cm-1) WTO= Transversal optical phonon (796cm-1) ε At high frequency SiC ~6.5  ε At low frequency SiC ~9.52 ε (0) ωL 2 LST relation: = 2 ε (∞) ωT 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.  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 ε1 ε2 σ (ω , q)i + = ε1ω 2 ε 2ω 2 ωε 0 q − 2 2 q − 2 2 c c Assuming we are in low q, so q<w/c, SPP dispersion relation is. ω2 1 q = 2 [1 − 2 ] c σ (ω , q ) ( + ε2 ) 2 450 ε 0c ω Free space dispersion relation is q = c Fig: SPP dispersion relation plot with free space dispersion 8 SPP dispersion intersects the free space dispersion -coupling of SPP into free space radiation- SiC substrate essential.
Clean Energy Lab (CEL) @ USC Viability of Plasmonics in Epitaxial Graphene q= wave vector Coupling between SPP and Single Particle Excitations ω= frequency•Intersection between SPP and free space ω1 = vF q •Coupling to free space•Intersection region has to be dominated byinterband 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. ω2 = 0 q < 2k F ω2 = γ q − 2 EF q > 2 k F 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 ‘ON’: When Ef is high, interband transitions allowed. interband transitions not Can transform plasmon to DC allowed. Can propagate signal current and vice-versa. without significant damping. Electrical manipulation of plasmonic signals.
Clean Energy Lab (CEL) @ USC Graphene Exfoliated graphene Epitaxial graphene ( single layer) (single or multi layer) Silicon (Si) GaAs 4H-SiC Metal Graphene (Ag)Supporting TE --- --- ---- No Yes modeDispersion Parabolic parabolic parabolic parabolic linear –EHP atrelation any wavelengthBand gap 1.12eV 1.42eV 3.23eV 0 0Electron Mobility <1400 <8500 <900 200000(cm2/v-s)RMS roughness --- ---- ------- ~1nm <0.5nmSPP Detection ----- ------ -------- Metal to Single materialand guiding guide, for guiding andmaterials 11 Semi to detection, detect  L A Falkovsky “Optical properties of graphene” . Phys.: Conf. Ser., Volume 129, Number 1 (2008)  M.Jablan, H. buljan, M. Soljacic “Plasmonics in Graphene at infrared frequencies” Phy.ReV. B 80 245435 (2009)
Clean Energy Lab (CEL) @ USCEpitaxial Graphene Growth Raman XPS & ARPES 6H-SiC GrapheneAB D peak (1345 cm-1)…..due to induced disorderCA G peak (1585cm-1)… due to in plane vibrationCB 2D peak (2670cm-1)…..due to doubleA resonant process A B C FiG: Realization of Graphene from 6H-SiC ID/IG…Disorder ratio <0.2  12  A.C Ferrari and J. Robertson “Interpretation of Raman spectra of disordered and amorphous carbon” Phys. Rev B 61 vol 61 num 20 (2000)  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 What Growth Growth mechanism is happens mechanism is defect&step in step flow mediated [**] between? mediated [*][*] M. Hupalo, E. Conrad, M. C. Tringides http://arxiv.org/abs/0809.3619[**] Appl. Phys. Lett. 96, 222103 (2010)
Clean Energy Lab (CEL) @USC 13000C 13500C 14000C 14500CSi faceA planeM planeC face
Clean Energy Lab (CEL) @USC Raman Characterization Si face C face All peaks are red shifted with increasing temp. What would a H2 etch do? 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  Experiment: Blank SiC is used as reference. ω 2 − ωLO + iΓ1ω 2 ε 2 = ε 2(ω ) = ε ∞ 2 ω − ωTO + iΓ 2ω 2 ∆2 (1 +) e 2 (ω + iΓ ) ∞ E2 σ int er ( ω ) = i ∫∆ dE ( 2 E ) 2 − ( hω + iΓ ) 2 [ f ( E − EF ) − f (− E − EF )] π e2 ∂f ( E − EF ) σ int ra ( ω ) = i π h 2 ∞ i ∫ −∞ dE ∂E ]E ω+ τ 2 ε1Nσ (ω) ×cos(Φ ( ε1ε 2ε 0 / α + ) c 1) −ε1ε 0Fig: Schematic view of FTIR differential R= 2reflection spectra setup ε1Nσ (ω) ×cos(Φ ( ε1ε 2ε 0 / α + ) c 1) +ε1ε 0 n1 1 − [( sin Φ1)]2 α= n2 cos Φ1 16  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 number of layer Fig: Variation of Fermi level 2 ε1Nσ (ω) ×cos(Φ ( ) ε1ε 2ε 0 / α + c 1) −ε1ε 0 R= 2 ε1Nσ (ω) ×cos(Φ ( ) ε1ε 2ε 0 / α + c 1) +ε1ε 0 Variable Parameter Number of Layer, N Fermi Energy Ef 17 Scattering time τ 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 ωLO 18 ωTO 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: •No of Layer N=2-17 •Fermi Energy Ef=10535meV •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 mathematicalmodel 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 / π ( hv F ) 2 Fig: Fermi level Vs No of layer 1 τ = k1( ) / vF ns 1 Short range scattering τ∝ ns Fig: Scattering time Vs avg. carrier density Coulomb scattering τ ∝ ns Mobility, µ= eτ vF / EF 2 20 Fitting value of k1=0.6 suggests our EG is Mobility (1000-10,000) cm /V-s 2 dominated by short-range scattering.  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-LONGClean Energy Lab (CEL) @ USC RANGE? Mirror Collecting Incoming •Pure N2 - inert gas light light signal source •15ppm NO2 -electron accepting gas •500ppmNH3 -electron donating gas Sensing element Graphene SiC Substrate SPP Graphene Fig: Experimental setupFindings: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 (ω + iΓ ) ∞ E σ int er ( ω ) = i ∫∆ ( 2E ) 2 − ( hω + iΓ ) 2 [ f ( E − EF ) − f (− E − EF )] dE π e2 σ int ra ( ω ) = i π h 2 ∞ dE ∂f ( E − EF ) ] E i ∫−∞ ∂E ω+ RPA approximation: τ e2 ns n F [4rs / (2 − π rs )] σ RPA T =0 = [ + i ] π h ni G[4rs / (2 − π rs )] 4ns Fig: Dielectric function of SiC Intraband-low f Interband high f 2π e2 2 2π sin θ 2 x2 (1 − cos θ ) 2 rs = 4πε 0ε SiC vF h G ( x) = x 8 ∫ θ dθ F ( x) = 8 ∫ θ dθ 0 (sin + x) 2 0 (sin + x) 2 2 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 Interband scattering Extracted parameter ni matters even at DC.
Clean Energy Lab (CEL) @ USCC-FACE IR REFLECTIVITY • Adsorbed molecules transfer charge charged scatterers • As ni increases, inter/intra band scattering increase • τ ~1/n i.e. i, 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 Fermi ni/ML Intra band Avg. Inter band Layer level (cm-2) scattering scattering (meV) time (fs) 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 0.3-0.5 22 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 0.1-0.2 9 N2 70 5.1x1011 10-20 15 3-4 NH3 90 5.5x1013 0.8-1 0.9 0.2-0.5 NO2 120 1.5x1014 0.4-0.5 0.45 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 conductivityElectron 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 Fermi ni/ML Intra band Avg. Inter band Layer level (cm-2) scattering scattering (meV) time (fs) time(fs) 34 N2 25 2x1011 90-280 185 27-60 NH3 30 6x1012 60-90 75 1.6-2 From FTIR NO2 35 2x1013 2-9 5 0.3-0.5 22 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 0.1-0.2 9 N2 70 5.1x1011 100-200 150 3-4 NH3 90 5.5x1013 0.8-1 0.9 0.2-0.5 NO2 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!
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?
ELECTROCHEMICALFUNCTIONALIZATION-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 RAMANSPECTROSCOPY 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 1200 D-peak red-shifts 1354-1335 cm-1. 1000 Raman Intensity (arb. units) G peak broadens and 800 slightly blue shifts ~3 cm-1 New peak at ~2930 600 2 Indicative of C- 400 Hbond G GraphaneD 200 D Graphene 0 1200 1600 2000 2400 2800 -1 Wavenumber (cm ) 34 • 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 Ratio between slope m of the linear background and the intensity of the G peak D peak m/I(G) Raman Intensity Measure of the bonded H content G peak Based on amourphous carbon S≈ 18µm results Wavenumber (cm-1) maybe dominated by grain Florescence is not seen in boundaries carbon only hydrocarbons!!!•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 ESTIMATEH-CONTENT Damage distinguished from functionalization by a) damage has unmesurable slope for a given D/G ratio b) D peak position 36
SUBSTRATE DEPENDENCE OFFUNCTIONALIZATION Table 1: Average Parameters From Each Substrate in StudySubstrate D-peak D-peak D/G D/G Normalized Normalized Position Position Ratio Ratio Slope Slope Before After Before After Before (µm) After(μm) (cm-1) (cm-1) SI(1°) 1348 1330 0.21 1.91 3.66 14.4SI2(on) 1344 1332 0.17 1.32 4.24 18.9SI3(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 OFFUNCTIONALIZATION WITH ANDWITHOUT PT NANOPARTICLES Chemically Deposited • Raman Shows: Platinum – Incredibly large D/G ratio~4.5 38 – Emergence of Fluorescence H2PtCl6 · 6H2O + DI water – Addition to D’ shoulder peak – C-H peak at ~2930
RESULTS OF EVAPORATED METALCATALYSIS 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 ID/IG Normalized Normalized Before After Ratio Ratio Slope Slope (cm-1) (cm-1) Before After Before (µm) After (µm)SI 1348 1330 0.21 1.91 3.66 14.4SI2 1344 1332 0.17 1.32 4.24 18.9SI3 1347 1331 0.13 0.6 3.93 4.42SI3 AuAvg 1342 1330 0.22 1.05 4.42 7.86SI3 PtAvg 1364 1330 0.086 1.24 3.81 17.69 Increased functionalization with metal catalyst 40 Increase in fluorescence bandgap?
SCANNING TUNNELINGSPECTROSCOPYK.M. Daniels, …MVS, R. Feenstra… et.al, presented at EMC2011accepted, 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