Outline. Introduction: graphene. Adsorption on graphene: - Chemisorption - Physisorption. Summary

Transcription

1 Outline Introduction: graphene Adsorption on graphene: - Chemisorption - Physisorption Summary 1

2 Electronic band structure: Electronic properties K Γ M v F = 10 6 ms -1 = c/300 massless Dirac particles! 2

3 Applications High speed electronics Transparent conducting layers: - touch screens - solar cells - LCD s nanosensors: - mass sensors - gas sensors - pressure gauge 3

4 Motivation Importance of adsorbates on graphene: Possibility of using graphene as a very sensitive sensor Inducing charge carriers in graphene Inducing a band gap Hydrogen storage Enhancement of spin relaxation time in hydrogenated graphene spin valve devices, M. Wojtaszek et al., arxiv:

5 Simulations Density Functional Theory (DF T): Simulation codes ABINIT, QUANTUM ESPRESSO and VASP periodic structures: 5

6 Physisorption-Chemisorption Adsorbates on graphene fall into 2 classes: adsorption distances: > 3 Å binding energies: mev low migration barriers adsorption distances: Å binding energies: > 0.5 ev high migration barriers Physisorption Chemisorption 6

7 Chemisorption

8 Chemisorption Adsorption of e.g. atomic hydrogen: sp 2 sp 3 Similar for other radicals such as: F, CH3, OH, NH2, [D. W. Boukhvalov, PRB (2008)] [T. O. Wehling, PRB (2009)] 8

9 Graphane 9

10 Chemisorption Complete hydrogenation and fluorination of a single layer Hydrogenation and fluorination of a bilayer: - Single side adsorption - Double side adsorption: formation of bilayer graphane or fluorographene 10

11 Graphane and fluorographene Different configurations: chair boat zigzag armchair 11

12 Stability analysis Graphane and fluorographene (Energies in ev) E b = E tot /n (E ads +E C ) E f = E tot /n (E mol /2+E C ) 12

13 Graphane and fluorographene Structural properties: fluorographene θcch θccc zigzag: armchair: 13

14 Graphane and fluorographene Electronic properties (chair) Elastic properties E gap (GGA) = 3.7 ev E gap (GW) = 6.1 ev Young s modulus E = 243 N/m Graphane E gap (GGA) = 3.2 ev E gap (GW) = 7.4 ev Fluorographene E = 226 N/m Experimental values for FG: E = 100 N/m [Nair et al., Small (2010)] E gap ~ 3 ev Defects 14

15 Graphene nanoribbons Son, Cohen, Louie, PRL 97, (2006) Armchair Zigzag Edge states 15

16 Graphene/graphane nanoribbons Studied by ab initio and tight-binding calculations 16

17 αα Graphene/graphane nanoribbons AFM AFM FM Non magn H pass. FM 17

18 αβ Graphene/graphane nanoribbons Not stable by H passivation, but stable in graphane Ferromagnetic semiconductor Edge state E g = 0.58 ev M=1 µ B /cell 18

19 ββ Graphene/graphane nanoribbons Not stable by H passivation, but stable in graphane Edge state Anti ferromagnetic ground state 19

20 Hydrogenation of a bilayer Single side adsorption (AB stacked bilayer) Similar to single layer graphene Small difference in E b for A and B sublattices: DE b = 0.03 ev (100 times smaller for adsorption with F) Maximum H coverage 25 at.% Boukhvalov et al., PRB (2008) 20

21 Hydrogenation of a bilayer Double side adsorption low H concentrations similar to single side adsorption 21

22 Hydrogenation of a bilayer Double side adsorption: high H concentrations Interlayer chemical bonding (ICB) 22

23 Hydrogenation of a bilayer How many H atoms are needed to induce ICB? 23

24 Hydrogenation of a bilayer Comparison of different hydrogenation schemes: single sublattice dimer A dimer B trimer (A+B) max. coverage: 25 at.% bilayer graphane 24

25 Physisorption

26 Ambipolar electric field effect: Electronic properties top gate V g S graphene D back gate 26

27 Physisorption Motivation: SiO 2 Si S graphene D Why graphene may be a good gas sensor? Graphene is 2D: only surface, no volume is highly conductive, metallic conductivity in the limit of zero carrier density has few crystal defects allows four probe measurements e - 27

28 Experimental results Adsorption of NH3, CO, H2O and NO2 1ppm Donors Acceptors Nature Materials 6, 652 (2007) 28

29 Experimental results Strongly diluted NO2 near neutrality point Ultimate sensitivity of one molecule for NO2 29

30 Computational approach DFT calculations for NH 3, NO 2, NO, CO and H 2 O on graphene using ABINIT Open shell spin polarized calculations 4x4 graphene supercell (32 C atoms) GGA underestimates bonding (LDA overestimates it) Three adsorption sites, different orientations Adsorption energy = energy isolated graphene + isolated molecule - fully relaxed graphene with adsorbed molecule Finite size effects (dipole-dipole interactions) mainly cancel Density of states Troullier Martins pseudopotentials, E cut =816 ev 5x5x1 Monkhorst-Pack grid For DOS: 15x15x1 Monkhorst-Pack grid and Gaussian smearing of 0.14 mev 16 Å between graphene layers 30

31 Charge transfer Hirshfeld charge analysis Electron density isolated atom Calculated density Adsorbate acts as donor or acceptor Comparison with experiment 31

32 Example: NH 3 on graphene HOMO LUMO NH3 is donor Determined by mixing HOMO with graphene orbitals LDA: Ea 100 mev Exp: 150 C 32

33 Non-magnetic molecules NH 3, H 2 O, CO NH 3 Adsorption distances: Å Binding energies: mev Charge transfers: to 0.05 e E F 33

34 Paramagnetic molecules NO, NO 2 5s * 2p * 2p 2p 1p 5s O N NO 34

35 NO 2 on graphene NO 2 is acceptor LUMO is below Dirac point Explains jumps in experiment Limited by finite supercell Adsorption of NO2 35

36 Summary: Donors Physisorption Acceptors adsorbate theory experiment E a (mev) DQ (e) H 2 O acceptor acceptor NH 3 donor donor CO donor donor NO 2 acceptor acceptor 67 ~ -1 36

37 Ti TiO 2 adsorption on graphene Motivation K. M. McCreary et al., Appl. Phys. Lett. 98, (2011) 37

38 Ti TiO 2 adsorption on graphene Ti adsorption MD simulations (1 ps, 500 K) Charge transfer methods fail to predict strong n-type doping 38

39 Ti TiO 2 adsorption on graphene Ti adsorption - Band structure - Work function Strong n-type doping 39

40 Ti TiO 2 adsorption on graphene TiO 2 adsorption Charge transfer calculations much more robust 40

41 Ti TiO 2 adsorption on graphene TiO 2 adsorption 41

42 Summary There are two distinct classes of adsorbates on graphene: Physisorption: 2 types of interaction with graphene: - Paramagnetic molecules large charge transfers - Nonmagnetic molecules small charge transfers Chemisorption: distortion of graphene lattice - Hydrogenation / fluorination new crystalline materials with large band gap - Graphene/graphane nanoribbons can be magnetic - Hydrogenation and fluorination of bilayer graphene 50 at.% coverage Ti-TiO 2 adsorption: tuning charge transfer in graphene 42