1 / 44

Clean Energy Lab (CEL)

Clean Energy Lab (CEL). Towards Plasmonics in Epitaxial Graphene M.V.S. Chandrashekhar Department of Electrical and Computer Engineering, University of South Carolina. USC G.Koley T.S. Sudarshan C. Williams J. Weidner B.K. Daas K.M. Daniels S. Shetu O. Sabih A. Obe. CMU

clint
Download Presentation

Clean Energy Lab (CEL)

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Clean Energy Lab (CEL) Towards Plasmonics in Epitaxial Graphene M.V.S. Chandrashekhar Department of Electrical and Computer Engineering, University of South Carolina 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

  2. 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

  3. 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 surfacesensitive to surroundings • Very transparent and highly conductive-touch screens?

  4. Clean Energy Lab (CEL) @ USC WHAT IS A PLASMON POLARITON? 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

  5. Clean Energy Lab (CEL) @ USC MOTIVATION: THE PLASMONIC CHIP • Overcome diffraction limit of light (d<λ/2) using SPP • Merge electronics and optics together in nano scaled range • Important for data processing, super lensing, sensing etc. Surface Plasmon Polariton at metal/dielectric interface SPP CHALLENGE: Couple Collective SPP to Single particle excitations When <0, K is imaginary Surface confinement [2] M. Dragoman, D. Dragoman, Nanoelectronics: Principles and Devices, Artech House, Boston, 2006

  6. Clean Energy Lab (CEL) @ USC HOW DO PLASMONICS WORK? • SPP propagation mediated by intra band processes • SPP detection mediated by inter band processes Graphene 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

  7. Clean Energy Lab (CEL) @ USC SIC SUBSTRATE DIELECTRIC FUNCTION WLO= Longitudinal optical phonon (972cm-1) WTO= Transversal optical phonon (796cm-1) At high frequency ~6.5 [8] At low frequency ~9.52 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. LST relation: [8] Dmitriy Korobkin, Yaroslav Urzhumov, and Gennady Shvets; J. Opt. Soc. Am. B, 23,3,468 (2006)

  8. 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 and When x<0 and Dispersion relation for TM mode is given by Assuming we are in low q, so q<w/c, SPP dispersion relation is. 450 Free space dispersion relation is 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.

  9. Clean Energy Lab (CEL) @ USC Viability of Plasmonics in Epitaxial Graphene Coupling between SPP and Single Particle Excitations q= wave vector = frequency • 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. Applying single particle excitation boundary condition for intra and inter band scattering Comes from graphene E-k bands (developed by S.Das Sarma)

  10. Clean Energy Lab (CEL) @ USC MODULATING EPITAXIAL GRAPHENEPLASMON WAVEGUIDE BY DOPING ‘ON’: When Ef is high, interband transitions not allowed. Can propagate signal without significant damping. ‘OFF’: When Ef is low, only interband transitions allowed. Can transform plasmon to DC current and vice-versa. Electrical manipulation of plasmonic signals.

  11. Clean Energy Lab (CEL) @ USC Graphene Epitaxial graphene (single or multi layer) Exfoliated graphene ( single layer) [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)

  12. Clean Energy Lab (CEL) @ USC Epitaxial Graphene Growth Raman 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)

  13. NON-POLAR FACE GROWTH-6H SIC EG on Si face EG on C face 5µm× 5µm 5µm× 5µm Growth mechanism is defect&step mediated [**] What happens in between? Growth mechanism is step flow mediated [*] [*] M. Hupalo, E. Conrad, M. C. Tringides http://arxiv.org/abs/0809.3619 [**] Appl. Phys. Lett. 96, 222103 (2010)

  14. Clean Energy Lab (CEL) @ USC 13000C 13500C 14000C 14500C Si face A plane M plane C face

  15. Clean Energy Lab (CEL) @ USC Raman Characterization Si face C face All peaks are red shifted with increasing temp. Decreasing stress with temperature increase 2D peaks narrow with increasing temperature What would a H2 etch do?

  16. Clean Energy Lab (CEL) @ USC Surface Plasmon Polariton (SPP) in Epitaxial Graphene Our approach Mathematical Model [7] Experiment: Blank SiC is used as reference. Fig: Schematic view of FTIR differential reflection spectra setup [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)

  17. 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 Variable Parameter Number of Layer, N Fermi Energy Ef Scattering time τ Fig: Variation of scattering time

  18. 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: IR reflection of SiC Substrate with SiC as reference Fig: AFM image of SiC Substrate Fig: IR reflection of EG with SiC as reference Fig: AFM image of EG (2ML)on SiC

  19. Clean Energy Lab (CEL) @ USC EG transport properties extraction using FTIR • Extracted Parameters: • No of Layer N=2-17 • Fermi Energy Ef=10535meV • 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

  20. Clean Energy Lab (CEL) @ USC EG transport properties extraction using FTIR B,K. Daas…MVS et al JAP (2012) Carrier density Fig: Fermi level Vs No of layer Short range scattering[9] Coulomb scattering[9] Fig: Scattering time Vs avg. carrier density Mobility, µ= Fitting value of k1=0.6 suggests our EG is dominated by short-range scattering. Mobility (1000-10,000) cm2/V-s [9] L A Falkovsky “Optical properties of graphene” . Phys.: Conf. Ser., Volume 129, Number 1 (2008)

  21. 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

  22. 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

  23. Clean Energy Lab (CEL) @ USC MOLECULAR DOPING OF EG-LONG RANGE? • Pure N2 - inert gas • 15ppm NO2 -electron accepting gas • 500ppmNH3 -electron donating gas Fig: Experimental setup Findings: Reflection amplitude changes -Looks like change of thickness but thickness can’t change

  24. Clean Energy Lab (CEL) @ USC Conductivity Matching: Optical Conductivity: RPA approximation: Intraband-low f Interband high f Fig: Dielectric function of SiC 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.

  25. 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 moleculeAgrees with Kelvin probe measurements

  26. Clean Energy Lab (CEL) @ USC

  27. 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)

  28. CORRELATION WITH KELVIN PROBE ~60% or more change in conductivity expected Scattering from impurities not enough to explain measured change in optical conductivity • Consistent with F.Schedin’s result of G/SiO2 • Assume ΔEf~10meV for 4ppm. μchem ill-defined. Electron affinity of NO2 dominates!

  29. Clean Energy Lab (CEL) @ USC 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!

  30. 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

  31. 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?

  32. PART II: FUNCTIONALIZATION

  33. 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 Before Scale: 8nm RMS: 1.00nm After Scale: 8nm

  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 D-peak red-shifts 1354-1335 cm-1. G peak broadens and slightly blue shifts ~3 cm-1 New peak at ~2930 Indicative of C-Hbond 2D G Graphane D Graphene • R. Sharma, et. al. Anomalously Large Reactivity of Single Graphene Layers and Edges toward Electron Transfer Chemistries, Nano Letters 10, 398-405 (2010)

  35. 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 • m/I(G) • Measure of the bonded H content • Based on amourphous carbon results • maybe dominated by grain boundaries D peak G peak Raman Intensity S≈ 18µm Wavenumber (cm-1) Florescence is not seen in 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.

  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 Table 1: Average Parameters From Each Substrate in Study * 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?

  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)

  40. SUMMARY: METAL CATALYSIS • Increased functionalization with metal catalyst • Increase in fluorescence  bandgap?

  41. 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?

  42. CYCLIC VOLTAMMETRY • Clear substrate dependence • Qualitatively different from bulk carbon • Clear peaks, not double-layer charging • Still investigating peak assignments

  43. 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

  44. 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

More Related