Electronic Properties of Hydrogenated Quasi-Free-Standing Graphene

Transcription

1 GCOE Symposium Tohoku University 2011 Electronic Properties of Hydrogenated Quasi-Free-Standing Graphene Danny Haberer Leibniz Institute for Solid State and Materials Research Dresden

2 Co-workers Supervising A. Grüneis (Vienna) T. Pichler (Vienna) M. Knupfer (IFW Dresden) B. Büchner (IFW Dresden) Experimental D. Vyalikh (TU Dresden) C. Giusca (Surrey, UK) D. Usachov (St. Petersburg, Russia) H. Sachdev (Mainz) R. Hübel / R. Schönfelder / S. Leger (IFF) Theoretical S. Taioli (Trento, Italy) B. Dora (Budapest, Hungary) M. Farjam (Tehran, Iran) S. A. Jafari (Tehran, Iran) Y. Wang, S. Irle (Nagoya, Japan)

3 Outline Introduction Part I: covalent functionalization Part II: ionic functionalization Part III: ionic + covalent ( complicated) Conclusions + Outlook

4 2D Carbon: Graphene sp² hybridized carbon atoms with σ-electrons (C-C bonding) and delocalized π electrons A. K. Geim et al., Nature (2007), Vol. 6 truly 2-dim. material with unique properties (discovered 2004)

5 Properties of Graphene So far 3 main routes to achieve Graphene: Mechanical exfoliation on SiOx Graphitization of SiC CVD synthesis on metal (111) substrates That s what we do massless Dirac particles with v F ~ 10 6 m/s anomalous Quantum Hall Effect Klein Paradox high charge carrier concentrations up to n ~ (e/cm²) high mobility µ ~ 15000cm²/(Vs) A. K. Geim et al., Nature (2007), Vol. 6

6 Energy Electronic structure of Graphene π band dispersion in the 1. BZ E hole K K k x ' k y ' electron k y k x linear! band dispersion near the Fermi level zero gap semi-conductor

7 Routes to a Band Gap in Graphene size confinement: chemical doping: width of 1nm corresponds to gap of ~1eV (depending on edge states) adding of functional groups, substitution of C-atoms Break symmetry between A, B carbon atoms C. Stampfer et al., Nano Lett. (2008), Vol. 8 D. Boukhvalov et al., PRB 77, (2008)

8 Chemisorption of atomic hydrogen fully saturated Graphene with sp 3 hybridization (diamond like structure) Bandgap of ~3eV Band insulator! J.O. Sofo et al., PRB (2007), Vol. 75 DFT calculations for partially hydrogenated Graphene 1. substantial gap at K opening 2. dispersionless Hydrogen acceptor level at E F 3. Spin splitting E.J. Duplock et al., PRL (2004), Vol. 92

9 Photoemission: Principle

10 ARPES: Principle ARPES: direct access to valence band properties, QP interactions, SINGLE CRYSTAL ONLY!!

11 ARPES: band structure explorer azimuth angle to set orientation (G-K) change polar angle to walk the line from G to K A. Bostwick et al., Nat. Phys. (2007), Vol. 3

12 (AR)PES in the Real World (at Synchrotron) BESSY II (Berlin) for XPS, ARPES ELETTRA (Trieste) for ARPES

13 Substrate preparation for Graphene CVD LEED Ni(111) thin film Deposition of ~100 Angstrom Ni W(110) single crystal (1 x 1 cm)

14 CVD growth of graphene and its functionalization A. Shikin, PRB (2000), Vol. 62 A. Varykhalov, PRL (2008), Vol Cracking of propylene on Ni(111) 2. Intercalation of Au into graphene/ni(111) Interface 3. Graphene layer bond to Ni is weakened A B

15 Graphene/Ni(111) vs. Graphene/Au/Ni(111) A. Grüneis et al., PRB (2008), Vol. 77

16 covalent functionalization: hydrogenated Graphene Energy [ev] K Momentum gap opening up when H applied! (consistent with theory) π band broadening ARPES provides a strategy to determine H-coverage: use ImΣ (lifetime) D. Haberer et al., Nano Lett. (2010), Vol. 10,

17 Energy Energy Momentum dispersion curves Momentum 0.8% 11% FWHM of MDCs continuously increasing k-k F

18 Energy Energy dispersion curves 0.8% Momentum pristine Graphene 0.5% H:C 5% H:C 17% H:C EDC maxima π band getting weaker depletion of PE Intensity at Fermi level (gap opening up) disorder induced MIT B. Dora et al. NJP 11, (2009)

19 ionic functionalization: n-doped Graphene rewind to Graphene/Au and insert additional electrons: Evaporation of potassium on Graphene/Au/Ni at ~30K warm up Intercalation of potassium between C/Au Interface

20 quasi-free-standing pristine and n-doped Graphene 0.5 Å -1 Graphene K Au Ni Å -1 π band shifted in energy after potassium doping, π* becomes visible

21 K-doping of Graphene/Au 0.1Å -1 K/C ~ 10% Intercalate K intercalated charge transfer to Graphene upon doping with K separation of π and π* after intercalation of K

22 Energy Energy dispersion curves EDC K Momentum K on top: π band shifted in Energy, small separation of π, π* (~300meV) K intercalated: distance increased (~800meV) electron-phonon coupling visible*

23 covalent + ionic functionalization increasing distance between VB and CB due to chemisorption of hydrogen Fermi surface becomes smaller upon H new Level appears D. Haberer et al., submitted

24 Energy EDC cuts of hydrogenated n-doped Graphene EDC Momentum

25 Summary and Conclusions tunable bandgap acceptor level q-f-s Graphene: H-level above E F, not populated, not visible n-doped Graphene: H-level below E F + in the gap, visible

26 Apply CVD route to h-bn D. Usachov, D. Haberer et al., PRB (2010), Vol. 82 CVD on metal substrates not only suitable for Graphene h-bn can be synthesized, functionalized and even equipped with Graphene

27 Valence Band Properties of (C/)h-BN BN/Ni C/BN/Au BN/Au

28 Thank You for your Attention!

29 Summary and Conclusions tunable bandgap acceptor level q-f-s Graphene: H-level above E F, not populated, not visible n-doped Graphene: H-level below E F + in the gap, visible