Impact of collective effects on charge transport through molecular monolayers

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1 Impact of collective effects on charge transport through molecular monolayers Obersteiner Veronika Institute of Solid State Physics

2 Outline Introduction Motivation Investigated Systems Methodology Results 2

3 Introduction What is molecular electronics? - single molecules as active elements in nanocircuits - based on the bottom-up approach - functional molecules add a unique ingredient of variability and structural control 3

4 Introduction A brief history of molecular electronics 1974 A.Aviram and M.Ratner - molecular rectifiers 1981 G.Binning and H.Rohrer - STM 1990 Mechanically Controllable Break-Junction MCBJ 1997 M.Reed and J.Tour - first transport experiment in single molecule junctions 4

5 Motivation molecular junction 5

6 Motivation single molecule vs. monolayer 6

7 Motivation - two isomeric molecules - same molecular IP expect same charge transport characteristics D. A. Egger et al, Adv. Mater. 2012, 24,

8 Motivation single molecule vs. monolayer 8

9 Motivation different transport characteristics as monolayer D. A. Egger et al, Adv. Mater. 2012, 24,

10 Motivation different charge transport polarity D. A. Egger et al, Adv. Mater. 2012, 24,

11 Transmission and Current polarity channel closest to Fermi Energy determines charge transport characteristics Proc. of SPIE Vol

12 Motivation D. A. Egger et al, Adv. Mater. 2012, 24, 4403 thermocurrent of opposite sign Seebeck coefficient of opposite sign 12

13 Motivation two isomeric molecules behave very different in monolayer devices due to collective electrostatic effects N in N out local dipoles add up no local dipoles D. A. Egger et al, Adv. Mater. 2012, 24,

14 Motivation fundamental difference in charge transport between single molecules and monolayers study collective effects through coverage dependent calculations in SAMs apply DFT combined with Green s function approach investigate different docking groups 14

15 Investigated Systems organic Au (111) π- conjugated system Au (111) 15

16 Investigated Systems - internal - docking - reference 16

17 Methodology Coverage dependent calculations - DFT using VASP [1] - periodic boundary conditions - PBE exchange-correlation functional - 3 layers of Au acting as slab [1] G. Kresse and J. Hafner, Phys. Rev. B 47, 558 (1993) 17

18 Methodology Geometry optimization - with VASP - conjugate gradient - optimized structure and length in z direction - relaxed innermost Au layers 18

19 Methodology Coverage dependence θ 1 θ θ

20 Methodology Transport calculations LE central region RE - DFT using SIESTA [1] - periodic boundary conditions - localized atomic orbitals - semi-infinite lead [1] José M. Soler et al 2002 J. Phys.: Condens. Matter

21 Transport calculations Landauer - Büttiker formalism I(V) 2e h T(E)[f L (E μ L ) f R (E μ R )]de μ T L/R E F trace[γ T(E, V) e L V 2 R G Γ T(E,0) R (G R ) t ] I V T f L/R μ L/R R G Γ L/R current voltage transmission function left/right electrode Fermi function left/rigth electrode chemical potential ret. Greens function broadening matrix 21

22 Results Coverage effect molecules with internal dipoles - internal - docking - reference 22

23 Coverage effect molecules with internal dipoles Nin vs. Nout - two isomeric molecules - same molecular IP (Ionisation Potential) - same docking chemistry 23

24 N out ΔE 0.5eV 24

25 N in ΔE 0.1eV 25

26 Coverage effect molecules with internal dipoles Nin Nout 26

27 Coverage effect molecules with internal dipoles Nin vs. Nout ΔE 0.8eV ΔE 0.2eV 27

28 Coverage effect molecules with internal dipoles Nin Nout two collective effects 28

29 Coverage effect molecules with internal dipoles Investigation molecule metal bonding 29

30 Molecule - metal bonding charge rearrangements upon bond formation Δρ ρ sys (ρ Au ρ mol ρ H ) 30

31 Molecule - metal bonding electrostatic potential step due to 2 V Δρ ε 0 Δρ Bond dipole 31

32 SAM-substrate bonding N out 32

33 SAM - substrate bonding N in 33

34 Coverage effect molecules with internal dipoles Nin Nout coverage effect of Bond Dipole similar 34

35 Coverage effect molecules with internal dipoles Nin Nout 35

36 Coverage effect molecules with internal dipoles Investigation molecule metal bonding free monolayer 36

37 Free monolayer coverage effect of local dipoles is different 37

38 Free monolayer Nin Nout local dipoles add up no local dipoles 38

39 Coverage effect molecules with internal dipoles two collective effects Nin local dipole and bond dipole effects cancel Nout local dipole and bond dipole effects do not cancel 39

40 Coverage effect molecules with internal dipoles - two isomeric molecules act very different as monolayers due to collective effects - ability of turning these collective effects off by tuning molecules with intramolecular dipoles 40

41 Transport calculations SIESTA localized atomic orbitals VASP plane waves compare DOS N out 41

42 Transport calculations SIESTA localized atomic orbitals VASP plane waves Nout 0.1 ev 42

43 Transport calculations SIESTA localized atomic orbitals VASP plane waves N in 43

44 Transport calculations SIESTA vs. VASP reproduce the trends, but for low coverages we still have a mismatch of about 0.1 ev localized atomic orbitals cannot describe vacuum very well introducing ghost atoms does not work so far 45

45 Transmission and Current full coverage I(V) 2e h T(E)[f (E μ L L ) f R (E μ R )]de 46

46 Transmission and Current full coverage I(V) 2e h T(E)[f (E μ L L ) f R (E μ R )]de 48

47 Transmission and Current full coverage 49

48 Transport calculations - coverage N out 50

49 Transport calculations Charge transport polarity change in charge polarity when going from monolayer to single molecule n-type p-type 51

50 Transport calculations - coverage N in 52

51 Coverage effect molecules with internal dipoles First conclusion N in Nout two molecules with same IP and docking chemistry result in two different monolayers due to collective effects ability of turning these collective effects off change in charge transport polarity when going from single molecule to monolayer due to collective effects 53

52 Results - internal - docking - reference 54

53 Results docking groups transport characteristics for full coverage L. A. Zotti et al, small 2010, 6, No.14, E. Lörtscher et al, ChemPhysChem 2011, 12, W. Hong et al, J.Am.Chem.Soc 2012, 134,

54 Results - internal - docking - reference 56

55 Reference System Tour CH2 SH different molecular IP same docking chemistry 57

56 Reference System Tour CH2 SH 58

57 Reference System Tour CH2 SH 59

58 N out N in Tour CH2 SH 60

59 Results - internal - docking - reference 61

60 Docking groups Tour Pyr different molecular IP different docking chemistry 62

61 Docking groups Tour Pyr LUMO pinned at the Fermi Energy only for full coverage 63

62 Docking groups Tour Pyr charge rearrangements extended along molecular backbone local polarization of the SAM 64

63 Docking groups Tour Pyr 66

64 Docking groups Tour Pyr 67

65 Summary collective effects are very important in molecular electronics two isomeric molecules result in two different monolayers with different charge transport polarity ability of turning collective effects off change in polarity when going from monolayer to single molecule 68

66 Summary docking chemistry strongly affects electrical transport collective effects can lead to Fermi level pinning 69

67 Outlook Clusters of molecules When do collective effects appear? 70

68 Acknowledgement supervisor: group: Egbert Zojer David, Iris, Gernot, Berni, Lisi and Elsi financial support: Austrian Science Fund 71

69 Thank you for your attention! 72

70 APPENDIX 73

71 DFT+ Σ Length Dependence of Conductance in Aromatic Single-Molecule Junctions S.Y. Quek, H. J. Choi,, S. G. Louie & J.B. Neaton, Nano Lett., Vol. 9(11), 2009,

72 DFT+ Σ Quantitative Current-Voltage Characteristics in Molecular Junctions from First Principles 75 P. Darancet, J.R. Widawsky,, H. J Choi,, L. Venkataraman & J.B. Neaton, Nano Lett., Vol. 12, 2012,

73 Isolated molecule vs. SAM Selzer, Y., Cai, L., Cabassi, M. A., Yao, Y., Tour, J. M., Mayer, T. S., & Allara, Nano letters, 5(1),

74 Experimental techniques R.L. Mc Creery, A.J. Bergen, Adv.Mater. 2009, 21,

75 MCBJ H.Song, M.Reed, T.Lee, Adv.Mater. 2011, 23,

76 MCBJ H.Song, M.Reed, T.Lee, Adv.Mater. 2011, 23,

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