FMM, 15 th Feb Simon Zihlmann

Transcription

1 FMM, 15 th Feb Simon Zihlmann

2 Outline Motivation Basics about graphene lattice and edges Introduction to Raman spectroscopy Scattering at the edge Polarization dependence Thermal rearrangement of the edge Summary

3 Motivation Graphene edge are responsible for many interesting features: band gaps, magnetism, ballistic transport Control of edges is difficult, but: Chemical unzipping of CNTs Mechanical cleaving of graphite Hydrogen plasma etching Thermal annealing for cleaning Hydrogen plasma etching at C

4 Lattice/edges and band structure a 1 = a 2 3, 3 b 1 = 2π (1, 3) 3a a 2 = a 2 3, 3 b 2 = 2π (1, 3) 3a a 1.42 A 60, 120, 180 same edge 30, 90, 150 different edge Castro Neto et al., Rev. Mod. Phys. 81, 2009

5 Raman spectroscopy Easy, non-destructive, non-contacting, quick method probing phonon/electron states Pumping with 532 nm, 2 mw Collecting all light with objective (NA=0.9) Spot size 360 nm Casiraghi, et al., Nano Lett. 9,

6 cm -1 Characteristic for sp 2 Doubly degenerate, LO and ito phonon mode Only one phonon process q~0 Independent of excitation wavelength Splits for pure Z-edge Malard et al., Phys. Rep. 473, 2009

7 cm -1 Defect induced Raman feature, or edges Dispersive, shape changes as well with laser energy Intervalley double resonance Raman process 1) e absorbs a photon, k (at K) 2) e scatters at a defect into k+q (at K, elastic) 3) e scatters back into k (at K, inelastic, q>>0) 4) Emitting photon by recombination with h Malard et al., Phys. Rep. 473, 2009

8 D 1620 cm -1 Defect induced Raman feature, or edges Dispersive, shape changes as well with laser energy Intravalley double resonance Raman process 1) e absorbs a photon, k (at K) 2) e scatters into k+q (at K, elastic) 3) e scatters back into k (at K, inelastic, q>>0) 4) Emitting photon by recombination with h Malard et al., Phys. Rep. 473, 2009

9 2D-band (or G 2680 cm -1 Second order process of D-band ( 2D) No defects necessary Two inelastic phonon scattering events (large q possible) Peak splitting Monolayer, 1 Bilayer, 4 Malard et al., Phys. Rep. 473, 2009; Ferrari et al., PRL 97, 2006

10 Scattering at the edge Backscattering for smooth edges only at normal incident, otherwise no radiative recombination (opposite momentum) Not true for disordered edge momentum selection Casiraghi, et al., Nano Lett. 9, 2009

11 Difference of Z- and A-edge d A connects K and K (intervalley) D-band active d Z does not connect K and K D-band inactive d A and d Z can lead to intravalley scattering D -band Casiraghi, et al., Nano Lett. 9, 2009

12 Polarization dependence Matrix element of e-h-pair (k, -k) creation is proportional to [e in x k] I(D) e in k 2 = e in 2 k 2 sin θ 2, max for θ = 90 k d A I(D) cos θ in 2 cos θ out 2 θ between edge and polarization I D θ in = I(D) min + I(D) max I(D) min cos (θ in θ max ) 2 Max in D-band with polarization along the edge

13 Experiment / Samples Monolayer graphene on SiO 2 Raman spectroscopy (D-band) Annealing in vacuum (5x10-5 mbar) at different temperatures ( C) Raman spectroscopy (D-band)

14 First hints of rearrangement 200 C 300 C, 15 min, 5x10-5 mbar 400 C 500 C P-doping due to vacuum annealing blue shift and sharpening of G-peak

15 Rearrangement microscopically ±60 zigzag reconstruction (Z60) ±30 armchair segments (A30) Point defects l c < l e ±60 armchair reconstruction (A60) ±30 zigzag segments (Z30) Point defects l c < l e f A30 2 cos θ cos θ f PD f A30 2 cos θ 2 + f A f PD f A0 cos θ 2 + f A60 2 cos θ cos θ f PD 1 2 2f A0 f A60 cos θ f A60 + f PD

16 Z-edge D-band weak, but nonzero edge not perfect but mainly Z0 (or Z60) segments At 200 C formation of first A30 segments (54%) Above 200 C, strong increase in A30 segments Above 300 C, angel dependence does not change, only overall intensity is growing more point defects f A30 2 cos θ 2 + f A f PD

17 A-edge Strong polarization dependence and clear D-band mainly A0 segments, fittings yields 83%, rest probably A60, only few point defects Intensity of D-band stays constant no significant contribution of Z30 Polarization dependence weaker more point defects but still 57% of A f A0 f A60 cos θ f A60 + f PD

18 Conclusion Edges are thermally unstable (above 200 C) Edge quality sinks Pure Z-edges form ±30 armchair segments Pure A-edges still dominated by armchair and no significant zigzag segments

19 Important for us Graphene edge characterization with polarized Raman spectroscopy is possible Thermal annealing (and current annealing) destroys zigzag (and armchair) edges Edge instability might be a driving force for the hydrogen etching continuous rearrangement of zigzag to armchair further etching Etching at low temperatures edge better? Splitting of G-band due to Z-edge