cule/électrodelectrode

Transcription

1 Interface molécule/ cule/électrodelectrode D. Vuillaume "Molecular Nanostructures & Devices" group Al Au Au hydrogen carbon oxygen sulfur silicon SiO2 1nm SiO2 1nm SiO2 1nm Si highly doped Si highly doped Si highly doped 1.0µm -(CH2)3-SH or C3-SH -(CH2)3-CH3 or C3 -(CH2)6-CH=CH2 or C8

2 Models and concepts Plan Energetics of molecules, films and interface Schottky-Mott model, Bardeen model, real interface, interface dipole Experimental techniques UPS, IPES, KP, photocurrent Some experimental results Physical origins and impacts on devices interface charge transfer, interface dipole CIGS, CNL, Fermi level pinning, IDIS "pillow"effect, work function modification effect of surface contamination organic on metal vs. metal on organic

3 Models and concepts

4 Energetics of molecules and films 0 E V E=eV=- e V

5 Energetics of molecule/metal Vondrak et al., J. Phys. Chem B (1999)

6 Schottky-Mott model Φ h =E F,HOMO S =1 Φ M Φ e = Φ M -EA Φ h = ΙΕ Φ M Schottky-Mott (1938)

7 Bardeen model Φ h =E F,HOMO CNL S =0 Φ M surface states fixed by surf. states metal does not matter Bardeen (1947)

8 real interface Φ h =E F,HOMO 0 S 1 Φ M breaking of the vacuum level alignment rule existence of an interface dipole

9 Φ e = S (Φ M - EA) + (1-S) (E G -CNL) Φ h = S (ΙΕ Φ M ) + (1-S) CNL 0 S 1 E G : gap HOMO-LUMO

10 Experimental techniques

11 HeI ev HeII 40.8 ev UPS-IPES

12 interface dipole Φ h =E F,HOMO

13 LUMO? : add the "transport gap" or measure it!

14

15

16 Kelvin Probe sample air vacuum probe CPD current flow oscillating probe : oscillating capacitance null current I pp =(CPD-V b ) C 0 ω (δsin(ωt + Φ)/d 0 ) Lord Kelvin (1861)

17 KPFM

18 photo-current hν>φ small V << Φ V << Φ : négligeable dark current (pulse light + lock-in amp detection) V>0

19 Φ e,al 4.3 ev ; Φ e,au 5 ev ; δφ comparable to δw M 0.9 ev Vuillaume et al., Phys. Rev. B (1998)

20 Some experimental results

21 IE measured by UPS

22 LUMO HOMO interface dipole Kahn et al. (2003)

23 Physical origins & Impacts on devices

24 Seki et al. (1999)

25 interface charge transfer typical case : Φ Μ < ΕΑ Φ M EA +q -q q = (ε i ε 0 /δ) (A mol /e) ε i and δ dielectric constant and thickness of the interface layer, A mol area per molecule. ε i ~ 3, δ ~ 0.5 nm, = 0.5 ev, A mol ~ 25 Ǻ 2 : q ~ 0.04 e - /mol

26 an example : PTCBI/Ag ~ 0.45 ev

27 chemically-induced induced gap states (CIGS) let us consider the case EA < Φ M <IE Φ M EA IE no charge transfer? no interface dipole?

28 it's experimentally NOT RIGHT!

29 depends on : - distribution of gap states - density of gap states In some cases : NOT dependent (or weakly) on the metal work function FERMI LEVEL PINNING

30 Example I : archetype system : Alq 3 /Mg HOMO CIGS organometallic complex

31 Example II : Al deposited on π-conjugated monolayers

32 Lenfant et al., J. Phys. Chem B (2006)

33 S ~ 0.025

34 consequence : not possible to tailor the rectification behavior by a chemical design

35 CNL : charge neutrality level How is this energy fixed? interface or gap states have charges + / 0 / - CNL : Q it = D it CNL

36 D it, S, Fermi level pinning (S~0) D it = ε 0 ε i (1-S) / eδs ε i and δ : dielectric constant and thickness of the "interface layer" Low S, D it larger than ~ cm -2 ev -1

37 IDIS : induced density of interface states weak chemical interaction : ex, Au/PTCDA Homo Lumo Flores et al., Europhysics Lett. (2004)

38 "pillow" effect adsorbed (physisorbed) molecules on metal µ naked metal : Φ M = µ + SD SD + physisorbed molecules : modify SD e-e repulsion reduces the electron tail and SD Φ M is reduced

39 α-npd on Au Φ M drops with the 1 rst monolayer

40 Φ M reduction hole injection barrier increase Better contact : large µ, small SD Example : conducting polymer such as PEDOT:PSS made of close-shell molecules far fewer free electrons than a metal no significant surface electron tail, small SD

41 similar effects with sexiphenyl, pentacene

42

43 Tailoring Φ M with molecule dipoles Φ = NµcosΘ/ε N dipole density (cm -2 ), µ dipole moment (D), Θ angle/surface normal, ε dielectric constant of monolayer (F/cm 2 )

44 1 Debye e 1 nm e

45 It also works on semiconductors GaAs EA X = OCH 3 ; CH 3 ; H ; CN ; CF 3 Cahen et al. J. Phys. Chem B (2005)

46

47 clean vs. dirty Au contamination : reduces metal electron tail decouples molecule and metal reduces IDIS, charge transfer and interface dipole Kahn et al., Org Electron. (2005)

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49 O on M vs. M on O Do the morphological differences influence the electronic structure of the interface and the device behavior?

50 Example I : CIGS dominate, answer is NO similar conclusions with Al and Sm

51 Example II : non-reactive interface, answer is YES Φ e,top - Φ e, bottom ~ ev

52 physisorbed vs. chemisorbed ε F V>0 tunnel current J= J 0 e β= α βd m* m 0

53 1.6 Physisorbed LB Decay factor (Ang -1 ) Au-S-R m*/m 0 =0.16 Si-R Energy barrier (ev) Si-O-Si-R "strong" coupling "weak" coupling E.E. Polymeropoulos et J. Sagiv, J. Chem. Phys. (1978) ; D. Vuillaume et al. Phys. Rev. Lett. (1996), Phys. Rev. B (1998) ; J. Holmlin et al., J. Am. Chem. Soc. (2001); D.J. Wold et al., J. Am. Chem. Soc. (2000,2001) ; X.D. Cui et al., J. Phys. Chem. B (2002); A. Salomon et al., Phys Rev Lett (2005)

54 influence of the chemical link Au(111)/mica 28 nm 0 Patrone et al., Chem Phys (2002)

55 Intensité (coups par seconde) Au/Se3 Au/T3 Au nu 0 2,0 1,5 1,0 0,5 0,0-0,5-1,0-1,5 Energie de liaison/e F (ev) S : HOMO-E F 0,5 ev Se: HOMO-E F 0,3 ev

56 En guise de conclusion et les autres interfaces? organique/organique organique/isolant

57 Bibliographie Articles de revues 1. Kahn, A.; Koch, N.; Gao, W., Electronic structure and electrical properties of interfaces between metals and piconjugated molecular films. J. Polymer Sci.: Part B: Polymer Phys. 2003, 41, Ishii, H.; Sugiyama, K.; Ito, E.; Seki, K., Energy level alignment and interfacial electronic structures at organic/metal and organic/organic interfaces. Adv. Mat. 1999, 11, (8), Cahen, D.; Kahn, A.; Umbach, E., Energetics of molecular interfaces. Materials Today 2005, July/August, Cahen, D ; Kahn, A., Electron energetics at surfaces and interfaces: concepts and experiments. Adv. Mater. 2003, 15(4), Vondrak, T.; Cramer, C. J.; Zhu, X.-Y., The nature of electronic coutact in self-assembled monolayers for molecular electronics: evidence of strong coupling. J. Phys. Chem. B 1999, 103, (42), Campbell, I. H.; Rubin, S.; Zawodzinski, T. A.; Kress, J. D.; Martin, R. L.; Smith, D. L.; Barashkov, N. N.; Ferraris, J. P., Controlling Schottky energy barriers in organic electronic devices using self-assembled monolayers. Phys. Rev. B 1996, 54, (20), Vazquez, H.; Oszwaldowski, R.; Pou, P.; Ortega, J.; Perez, R.; Flores, F.; Kahn, A., Dipole formation at metal/ptcda interfaces: Role of the charge neutraliy level. Europhys. Lett. 2004, 65, (6), Lenfant, S.; Guerin, D.; Tran Van, F.; Chevrot, C.; Palacin, S.; Bourgoin, J.-P.; Bouloussa, O.; Rondelez, F.; Vuillaume, D., Electron transport through rectifying self-assembled monolayer diodes on silicon : Fermi level pinning at the molecule - metal interface. J. Phys. Chem. B 2006, 110, (28), Wan, A.; Hwang, J.; Amy, F.; Kahn, A., Impact of electrode contamination on the a-npd/au hole injection barrier. Org. Electron. 2005, 6, Crispin, X.; Geskin, V.; Crispin, A.; Cornil, J.; Lazzaroni, R.; Salaneck, W. R.; Brédas, J. L., Characterization of the interface dipole at organic/metal interfaces. J. Am. Chem. Soc. 2002, 124, (27), Haick, H.; Ghabboun, J.; Niitsoo, O.; Cohen, H.; Cahen, D.; Vilan, A.; Hwang, J.; Wan, A.; Amy, F.; Kahn, A., Effect of molecular binding to a semiconductor on metal/molecule/semiconductor junction behavior. J. Phys. Chem. B 2005.