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

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

5. V Cu e-carbon Packaged: FIB/TEM: “modified electrode” Ox Red Cu Si
silicon (for contrast) Cu Si
10 nm
BTB
PPF
e-C Au
e-carbon
“modified electrode”
Ox Red
1-6 nm
sp2 carbon
Adv. Mater , 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. Mag. = 10 PPF Echip 4’’ wafer Mag. = 10 Magnification = 1 Junction
molecules attached to PPF by electrochemical reduction of diazonium ions
PPF leads
Junction
Mag. = 10
PPF Echip 4’’ wafer
Cu/Au deposited
through shadow mask
Mag. = 10
next
slide
Magnification = 1

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

9. near Jasper, Alberta

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

25. the usual suspects: characteristic plot:
Fowler Nordheim (field emission) linear lnJ vs 1/E (E= electric field)
variable range hopping linear lnJ vs T-1/2 or T-1/4
Schottky emission (i.e. thermionic) linear lnJ vs 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
(note: controlled by electric field,
not thickness. certainly NOT tunneling)
R2 =
300 K
4.5 nm
8.0
10.5 22
< 10K
4.5 nm 22
10.5
8.0

26. ( ) Poole-Frenkel transport between “coulombic traps”: V < 0 e-
trap depth decreases
with increasing electric field
Arrhenius slope (ln J vs 1/T) =
ϕtrap E1/2 q πε
( )
1/2
note: a field-dependent “activation” barrier

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

28. molecular orbital (e.g. HOMO):
Suppose the “trap” is
actually an occupied
molecular orbital (e.g. HOMO):
BTB subunits, connected but not polarons
~0.35 V “trap”
(i.e. HOMO)
V=0 +
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”

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

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

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