Hydrogenated Graphene

Transcription

1 Hydrogenated Graphene Stefan Heun NEST, Istituto Nanoscienze-CNR and Scuola Normale Superiore Pisa, Italy

2 Outline Epitaxial Graphene Hydrogen Chemisorbed on Graphene Hydrogen-Intercalated Graphene

3 Outline Epitaxial Graphene Hydrogen Chemisorbed on Graphene Hydrogen-Intercalated Graphene

4 Graphene growth on SiC(0001) ML Buffer Layer Buffer Layer SiC SiC Buffer Layer Topologically identical atomic carbon structure as graphene. Does not have the electronic band structure of graphene due to periodic sp 3 C-Si bonds. F. Varchon, et al., PRB 77, (2008). Theoretical Calculations 6 3 x 6 3 F. Varchon, et al., PRB 77, (2008). Superstructure of both the buffer layer and monolayer graphene on the Si face from the periodic interaction with the substrate.

5 Buffer Layer Bias: +1.7V Current: 0.3nA quasi-(6x6) F. Varchon, et al., PRB 77, (2008) Å 2.0Å 2.00 S. Goler et al.: Carbon 51, 249 (2013).

6 6 3x6 3-Superstructure Graphene SiC 30 nm, 1V, 100 pa E= 75 ev

7 Monolayer Graphene STM S. Goler et al.: J. Phys. Chem. C 117, (2013).

8 Hydrogen Intercalation Buffer Layer ML SiC SiC Buffer Layer (BL) Quasi-free standing monolayer graphene Quartz tube P ~ atmospheric pressure T ~ 800 C H 2 C. Riedl, C. Coletti et al., PRL 103, (2009)

9 Energy (ev) Energy (ev) Hydrogen Intercalation Buffer Layer ML SiC SiC E F = Buffer Layer (BL) Quasi-free standing monolayer graphene E F = k (Å -1 ) S. Forti, et al., PRB 84, (2011). k (Å -1 ) p= cm -2

10 S. Goler et al.: Carbon 51, 249 (2013). BL vs. QFMLG

11 S. Goler et al.: Carbon 51, 249 (2013). BL vs. QFMLG

12 Outline Epitaxial Graphene Hydrogen Chemisorbed on Graphene Hydrogen-Intercalated Graphene

13 Band Gap 3.5 ev Science 323 (2009) 610

14 Gap vs. H Coverage E gap = 3.8 ev cov 100% 0.6 A. Rossi et al., J. Phys. Chem. C 119, 7900 (2015).

15

16 H-dimers and tetramers Para-dimer Ortho-dimer Tetramer S. Goler et al.: J. Phys. Chem. C 117, (2013).

17 STS after hydrogenation S. Goler et al.: J. Phys. Chem. C 117, (2013).

18 H adsorption and desorption S. Goler et al.: J. Phys. Chem. C 117, (2013).

19 RMS roughness S. Goler et al.: J. Phys. Chem. C 117, (2013).

20 DFT calculations S. Goler et al.: J. Phys. Chem. C 117, (2013).

21 Graphene for hydrogen storage Graphene is lightweight, inexpensive, robust, chemically stable Large surface area (~ 2600 m 2 /g) Functionalized graphene has been predicted to adsorb up to 9 wt% of hydrogen Lee et al., Nano Lett. 10 (2010) 793 Durgen et al., PRB 77 (2007)

22 Graphene Curvature Exploit graphene curvature for hydrogen storage at room temperature and pressure The hydrogen binding energy on graphene is strongly dependent on local curvature and it is larger on convex parts Atomic hydrogen spontaneously sticks on convex parts; inverting curvature H is expelled Hydrogen clusters on corrugated graphene V. Tozzini and V. Pellegrini: J. Phys. Chem. C 115, (2011).

23 Graphene Curvature Exploit graphene curvature for hydrogen storage at room temperature and pressure The hydrogen binding energy on graphene is strongly dependent on local curvature and it is larger on convex parts Atomic hydrogen spontaneously sticks on convex parts; inverting curvature H is expelled Hydrogen clusters on corrugated graphene V. Tozzini and V. Pellegrini: J. Phys. Chem. C 115, (2011).

24 Graphene Curvature Exploit graphene curvature for hydrogen storage at room temperature and pressure The hydrogen binding energy on graphene is strongly dependent on local curvature and it is larger on convex parts Atomic hydrogen spontaneously sticks on convex parts; inverting curvature H is expelled Estimated GD: ~8 wt% V. Tozzini and V. Pellegrini: J. Phys. Chem. C 115, (2011).

25 Outline Epitaxial Graphene Hydrogen Chemisorbed on Graphene Hydrogen-Intercalated Graphene

26 Introduction Graphene on silicon carbide (SiC) (0001) buffer layer annealing epitaxial monolayer graphene (EMLG) H intercalation quasi-free-standing monolayer graphene (QFMLG) C. Riedl, C. Coletti, T. Iwasaki, A. A. Zakharov, and U. Starke, Phys. Rev. Lett. 103, (2009)

27 EMLG 10 3 The carrier mobility of QFMLG shows less temperature dependence than EMLG, indicating less interaction between QFMLG and the SiC substrate. However, the mobility of QFMLG (-3000 cm 2 V -1 s -1 ) is still lower than exfoliated graphene on SiO 2 or free standing graphene. F. Speck, J.Jobst, F. Fromm, M. Ostler, D. Waldmann, M. Hundhausen, H. B. Weber, and Th. Seyller, Appl. Phys. Lett. 99, (2011)

28 hole density cm -2 S. Tanabe, M. Takamura, Y. Harada, H. Kageshima, and H. Hibino, Jpn. J. Appl. Phys. 53, 04EN01 (2014). The QFMLG mobility depends on T H, the substrate temperature during H intercalation highest mobility by T H = 700ºC conductivity carrier density linear for T H = ºC - charged impurity sublinear for T H = 950ºC - additional scattering by defect Purpose : to observe the morphology of QFMLG formed at different T H and investigate the relationship with transport property

29 Intercalation at ºC T H = 800ºC 50 nm 8 nm width: 1.5 nm depth: pm honeycomb inside, no defect Y. Murata et al., Appl. Phys. Lett. 105, (2014).

30 Intercalation at ºC T H = 800ºC 1.8 nm STM of a buffer layer corrugation 60 pm <1120> width: 1.5 nm depth: pm honeycomb inside, no defect align along SiC<1120> periodicity: 1.8 nm = SiC nm 8 nm Y. Murata et al., Appl. Phys. Lett. 105, (2014). H SiC substrate Goler, Carbon 51, 249 (2013) incomplete H intercalation Si dangling bond at interface

31 Voltage [mv] Si dangling bond A dat Ch: 1 Biasvoltage: V Current: 1.0E-11A Temperature: [K] STS at 4K provides further evidence for dangling bonds, cf. F. Hiebel et al., PRB 86, (2012) nm 1.8 Aux channels nm nm nm nm nm Voltage [mv] Collaboration with Gerd Meyer, IBM Rueschlikon

32 Intercalation at 1000ºC 200 nm 50 nm 8 nm 0.25 nm ~ single layer of SiC large dark spots (width: 4-10 nm, depth: 0.25 nm)

33 Intercalation at 1000ºC 200 nm 50 nm 8 nm - width: 4-10 nm, depth: 0.25 nm 0.25 nm ~ single layer of SiC - distribute randomly - honeycomb inside hole in the SiC substrate etched at high T H SiC substrate

34 Wrinkles of graphene AFM images differentiated in horizontal axis 14 μm T H = 650ºC 800ºC 950ºC 1100ºC TEM T H = 1200ºC wrinkles appear at T H > 800ºC more pronounced at T H > 1100ºC the difference in thermal expansion coefficients between graphene and SiC

35 Morphology and transport T H = ºC H small dark spots SiC substrate incomplete H intercalation - Si dangling bonds ARPES Forti, et.al., Phys. Rev. B 84, (2011). annealing QFMLG in vacuum -H atoms desorb -Si dangling bonds donate charge to graphene and act as charged scattering centers.

36 Morphology and transport T H = 1000ºC dark spot hole in SiC substrate wrinkle of graphene SiC substrate Scanning tunneling potentiometry Ji, et.al., Nature Materials 11, 114 (2012) resistance of EMLG increases over SiC substrate steps π-σ hybridization by curvature of graphene strain of graphene reduced doping due to a larger distance at the interface T. Low, V. Perebeinos, J. Tersoff, and Ph. Avouris, Phys. Rev. Lett. 108, (2012)

37 Morphology and transport As T H increases from to 1000ºC, small dark spot decreases. -more H intercalation hole in SiC substrate and wrinkles of graphene appear The formation of holes and wrinkles has a larger influence on the QFMLG mobility than increased H intercalation. S. Tanabe, Jpn. J. Appl. Phys. 53, 04EN01 (2014).

38 S. Goler Y. Murata T. Mashoff C. Coletti V. Tozzini V. Piazza P. Pingue F. Colangelo V. Pellegrini F. Beltram M. Takamura S. Tanabe H. Hibino K. V. Emtsev, U. Starke, S. Forti

39 Funding