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University of Alberta National Institute for Nanotechnology Edmonton, Alberta, Canada

Molecules in Circuits, a New Type of Microelectronics? Richard L. McCreery. University of Alberta National Institute for Nanotechnology Edmonton, Alberta, Canada. “Organic Electronics”:. Sony organic LED display. charge transport by “hopping” across 100 nm – 10 μ m distances

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University of Alberta National Institute for Nanotechnology Edmonton, Alberta, Canada

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  1. Molecules in Circuits, a New Type of Microelectronics? Richard L. McCreery University of Alberta National Institute for Nanotechnology Edmonton, Alberta, Canada

  2. “Organic Electronics”: Sony organic LED display • charge transport by “hopping” across 100 nm – 10 μm distances • carriers are radical ions and/or “polarons”, often low stability What happens when charge transfer distance is decreased to 1-25 nm, and electric fields may exceed 106 V/cm ?

  3. 50 µm 50 µm Our substrate: Pyrolyzed Photoresist Films (PPF) commercial photoresist (novolac resin) ‘Soft Baking’ 90OC 6000 rpm Clean Si Wafer Spin-Coating Optional Lithography “PPF” (roughness < 0.4 nm rms) Pyrolysis Gas Flow 95% N2;5% H2 (1000OC, 1 hour) Kim, Song, Kinoshita, Madou, and White, J. Electrochem. Soc., 1998, 145, 2315 Ranganathan, McCreery, Majji, and Madou, J. Electrochem. Soc., 2000. 147, 277–282 Ranganathan, McCreery, Anal. Chem,2001, 73, 893-900.

  4. R R R e + CH3CN R HNO2 R NH2 + N PPF electrode 2 • N2 + sp2 carbon Delamar, M.; Hitmi, R.; Pinson, J.; Saveant, J. M.; J. Am. Chem. Soc.1992, 114, 5883. Liu, McCreery, J. Am. Chem. Soc., 1995, 117, 11254. Belanger, Pinson, Chem. Soc. Reviews 2011, 40, 3995 . Surface modification via diazonium reduction: e- • surface bond stable to > 500oC • high coverage • very low in pinholes • prone to multilayer formation.

  5. V “modified electrode” Ox Red FIB/TEM: Au Packaged: Cu e-carbon 10 nm Cu silicon (for contrast) Si 1-6 nm Au e-C BTB sp2carbon PPF Adv. Mater. 2009, 21, 4304 J. Phys. Chem. C 2010, 114, 15806 J. Am. Chem. Soc. 2011, 133, 19168

  6. Pyrolyzed Photoresist Film (PPF) microfabrication: 1. lithography 2. pyrolysis 100 mm PPF structure and conductivity similar to glassy carbon roughness < 0.5 nm rms by AFM

  7. PPF Echip 4’’ wafer molecules attached to PPF by electrochemical reduction of diazonium ions Cu/Au deposited through shadow mask next slide Junction Magnification = 1 Mag. = 10 Mag. = 10 PPF leads

  8. Mag. = 60 FIB/TEM: Au Mag. = 5,000,000 cleave through junction region, then SEM of “edge” TEM Cu silicon (for contrast) 5 nm sp2 carbon Cu/Au Cu/Au 100 nm SiO2 SiO2 PPF SiO2 Molecules (not resolved) PPF Si 1 µm junction region Mag. = 300,000 Mag. = 30,000

  9. near Jasper, Alberta

  10. no molecule 80 x 80 μm junction 2 32 junctions 1 mA O O 1 N ) 2 NAB (nitroazobenzene) J(A/cm N 0 N NAB 0.5 V -1 V -2 -1.0 -0.5 0.0 0.5 1.0 • frequency independent, • 0.01 to 105 Hz • symmetric • can scan > 109 cycles without change • survived 150 oC for > 40 hrs log scale looks like Marcus kinetics Is it? exponential above ~0.2 V V ACS Applied Materials & Interfaces 2010, 2, 3693

  11. Arrhenius: 37 meV T = 5 K 0.03 meV 1000/T, K-1 molecular layer thickness 3.3 nm • not activated for T<200 K 4.5 nm • strong thickness dependence 5.2 nm • NOT activated electron • transfer, so what is it? d = 2.2 nm 2.8 nm J. Phys. Chem C, 2010, 114, 15806

  12. molecules, insulator, etc Tunneling: tunneling barriers: LUMO ϕelectron Cu e¯ sp2 carbon eˉ EFermi vacuum ϕhole electron density d HOMO J (A/cm2) = K exp(-βd) 4 metal, β ~ 0 2 0 NAB: β = 2.5 nm-1 now, vary HOMO and LUMO energies ln J (0.1) -2 -4 alkane, β = 8.6 nm-1 -6 vacuum: β ~ 25 nm-1 -8 0 1 2 3 4 5 6 d, nm

  13. BTB More molecules: 2.4 eVrange of HOMOs, 2.3 eV range of LUMOs S S NO EFFECT! also: biphenyl, nitrophenyl, ethynyl benzene, anthraquinone (9 molecules, >400 junctions) O O N N N N N NH β = 8.7 nm-1 Sayed, Fereiro, Yan, McCreery, Bergren; PNAS (2012) 109, 11498.

  14. vacuum Energy levels of PPF/molecule from Ultraviolet Photoelectron Spectroscopy: e‾ e‾ WF hv=21.2 eV 4.53 WF=4.40 4.97 4.88 EFermi Br NO2 PPF EF - EHOMO * CH C mean for 8 aromatics= 1.3 ± 0.2 eV aliphatic= 2.0 ± 0.1 eV * method of Kim, Choi, Zhu, Frisbie, JACS 2011, 133, 19864

  15. -2 Molecules bonded to PPF: Free molecule HOMOs: -3 -4 0.69 Energy (eV vs. Vacuum) -5 1.3 ± 0.2 eV (aromatics) BTB -6 AB 2.0 ± 0.1 eV PhBr NC8H17 2.99 PPF -7 NAB UPS and Simmons barriers are consistent, and correlate with observed β values NP NC8H17 -8 • Sayed, Fereiro, Yan, RLM, Bergren, PNAS, 109, 11498 (2012)

  16. “leveling” effect of strong coupling • need to consider the system, not just • isolated components Why? WF ~ 4.4+eV 4.6 eV+ 1.2 eV WF ~ 4.6+eV Free molecules: S S HOMO ~5.3* 0.7 eV S S electron donating N O 2.0 N N 4.6 eV O N O WF ~ 5.1+eV N N PPF (unmodified) O 1.3eV HOMO ~6.6* electron withdrawing * B3LYP 6-31G(d) + experimental

  17. HOMO-1 orbital* Strong coupling: “graphene” electrode 0o dihedral 20o dihedral 90o dihedral 37o dihedral 60o dihedral dihedral angle between G9 and NAB “molecule” Cu • Substituents on molecule • affectboth HOMO and Fermi levels in • “strong coupling” limit • Where does the electrode • stop and the molecule begin? *Gaussian ‘03, B3LYP/6-31G(d)

  18. Something special about molecular tunnel junctions: 5 nm electron transit time ≈ 15 fsec (15 x 10-15seconds) eˉ molecules maximum frequency > 10,000 GHz whatever we can do with tunnel junctions should be VERY fast 0 V +1 V

  19. a problem with tunneling: J (A/cm2) = K exp(-βd) β = 0.03 nm-1 (??) 1 0.9 0.8 0.7 0.6 9 probability 0.5 3 0.4 β = 1 nm-1 (??) 1 0.3 0.03 0.2 0.1 β = 9 nm-1 (alkane) 0 0 1 2 3 4 5 distance, nm β = 3 nm-1 (aromatic) we need to seriously reduce β if molecular electronics is to be broadly practical so far, we are really doing “barrier” electronics

  20. BTB S S ϕe (electron tunnelling barrier) Efermi ϕh metallic contacts (hole tunnelling barrier) d β = 8.7 nm-1 what happens beyond tunnelling?

  21. BTB (bis-thienyl benzene) in “all-carbon” junction S Energy vs. vacuum, eV S S S S S S S S Au S S S S S S S e-beam carbon -1.5 eV V S S S S S S S S S S S S S S 4.5 – 22 nm multilayer -4.8 metallic contacts -5.3 PPF (sp2 carbon) collaboration with: Maria Luisa Della Rocca, Pascal Martin, Philippe Lafarge, Jean-Christophe Lacroix, University of Paris

  22. thickness, nm: 8.0 16 22 5.0 4.5 13 tunnelling? unlikely across 22 nm 300 K 16 13 10.5 4.5 5.0 7.0 8.0 note curvature, not simple exponential 22 conventional wisdom: activated “hopping” by redox exchange for d > 5-10 nm 300 K

  23. 8.0 10.5 4.5 nm 22 300 K 8.0 4.5 nm 22 <10 K 10.5 weakly activated, if at all, not consistent with redox exchange

  24. What about β, the attenuation coefficient ? β = 3.0 ± 0.3 nm-1 (N=8) 300 K 6K coherent tunnelling for d< 8 nm 100K 10 200K β = 1.0 ± 0.2 nm-1 (N=19) so what is this? not activated, β~ 1 nm-1 5 250K 300K 0 activated hopping, d> 15 nm, T>200 K ln J (1V) β = 0.1 ± 0.1 nm-1 (N=6) at 300K -5 -10 -15 Arrhenius slope= 160 meV (>200 K) 0.3 meV (5-50 K) -20 0 5 10 15 20 25 d , nm (bulk polythiophene: 130 – 280 meV) Haijun Yan, et al. PNAS, 110, 5326 (2013)

  25. the usual suspects: characteristic plot: Fowler Nordheim (field emission) linear lnJvs 1/E (E= electric field) variable range hopping linear lnJvs T-1/2 or T-1/4 Schottky emission (i.e. thermionic) linear lnJvs 1/T redox hopping linear ln J vs 1/T space charge limited conduction linear J vs V2 Poole-Frenkel transport between “traps” linear ln(J/E) vs E1/2 R2= 0.9989 (note: controlled by electric field, not thickness. certainly NOT tunneling) 300 K < 10K 4.5 nm 4.5 nm 8.0 8.0 10.5 22 10.5 22

  26. trap e- Poole-Frenkel transport between “coulombic traps”: V << 0 V < 0 V=0 'trap e- e- 'trap trap depth decreases with increasing electric field (ϕtrap- q ( ) 1/2 ϕtrap - E1/2 Arrhenius slope (ln J vs 1/T) = πε note: a field-dependent “activation” barrier

  27. 350 200 - 300K trap 16 nm 300 250 200 22 nm 150 Arrhenius slope, meV 100 10.5 nm 50 0 0 500 1000 1500 2000 E1/2(V/cm)1/2 1/2 E activation barrier → 0 at high field q ( ) 1/2 ϕtrap - E1/2 Arrhenius slope (ln J vs 1/T) = πε

  28. BTB subunits, connected but not polarons Suppose the “trap” is actually an occupied molecular orbital (e.g. HOMO): ~0.35 V “trap” (i.e. HOMO) V=0 “Molecular field ionization” Note: field ionization in gas phase occurs at ~107 V/cm; our field is ~106 V/cm, but “trap” is much shallower - A bit like “dry electro- chemistry”, but without a double layer and not “activated” + V<0

  29. Richard Feynman, 1959 “There's Plenty of Room at the Bottom” β = 2.7 nm-1 β = 8.7 nm-1 “barrier electronics” and not much control over barrier height “the bottom” for aromatic molecular junctions ?

  30. bulk semi- conductors tunnel junctions transport distance, nm 0 10 30 40 100+ nm 20 coherent tunnelling conjugation length ?? field ionization scattering length ?? resonant transport hopping, redox exchange (low mobility, reactive species) There is plenty of room in the middle !! Phys. Chem. Chem. Phys., 2013,15, 1065

  31. A preview of some new molecules: β≈ 3 nm-1 ϕe 0 β≈ 0.3 nm-1, Eact < 30 meV NAB, AB, etc -5 Efermi ϕh β≈ 0.12 nm-1, Eact < 50 meV metallic contacts -10 BTB factor of >106 -15 β≈ 0, but thermal -20 d, nm 0 10 20 30 β≈ 1 nm-1

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