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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 surfacesensitive 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 i1 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 i1 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=10535meV 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 40 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