Electron Transport in Strongly Coupled MolecularElectronic Junctions

Richard McCreery, Adam Bergren, Sergio JimenezBryan Szeto, Jie

Ru, Andriy

Kovalenko, Stan Stoyanov

University of AlbertaNational Institute for Nanotechnology

National Institute for Nanotechnology

University of Alberta,Edmonton, Alberta,Canada

Founded 2002Building dedicated 2006

$$$:

National Research CouncilNSERC (Canada)Alberta Ingenuity FundCanada Fund for Innovation

“Molecular electronics”“molecularjunctions:”

Today’s question: How are electrons transported through molecules ??

Note:• molecules become circuit elements• critical dimension is 1-10 nm

E=0 V

E=-0.7

E vs. NHE

Electrochemical reduction:

+0.5

-0.5

0

distance from electrode

LUMO

HOMO

-1.0

+1.0

ionic double layer

+

-

+

-+

-

+

-

+

-+

--

-

-

+

+

molecule insolution

metal electrode

ener

gy re

lativ

e to

vac

uum

, eV

-7

-6

-4

-5

distance

metalliccontacts

Efermi

Molecular junction:

two electrodes, no doublelayer, no solution

ener

gy re

lativ

e to

vac

uum

, eV

Two common electron transport models:

“off-resonant”

e.g. tunneling, Schottky,field emission

-7

-6

-4

-5

distance

metalliccontacts

Efermi

“resonant”

e.g. “resonant tunneling”,“orbital mediated tunneling”

-7

-6

-4

-5

metalliccontacts

LUMO is close (~within kT)to Fermi level

HOMO

Efermi

-+ e-

LUMO

HOMO

-+ e-

The scientific question:

How are electrons transported through 1-5 nm thick molecular layers?

Outline: • fabrication of molecular junctions• characterization• electronic properties• transport mechanism

V

Cu

Au

Things we do differently fromeveryone else:

sp2

carbonvery flat (< 0.5 nm rms)graphitic carbon substrate[Pyrolyzed Photoresist Film, PPF,essentially metallic, with ρ=0.006 Ω-cm]

covalent C-C surfacebond, stable to > 500 oC

conjugated, partially orderedmono-

or multilayer, 1-5 nm thick,108

– 1012

molecules in parallel

slow electron beam deposition ofCu top contact, often covalentlybonded to molecule

Phys. Chem. Chem. Phys, 2006, 8, 2572J. Chem. Phys. 2007, 126, 024704

next slide

J. Phys. Cond. Matter, 20, 374117 (2008)

1 µmmolecular layer (not resolved)

SEM TEM

20 nm

polypyrrole

carbon

metal

EMmount

PPF Echip

4’’

wafer PPF Echip

Clip electrodeused for electro-chemistry

PPF leads

Junction

Microfabricated

“E-chips”

cut aftermoleculedeposition

Bryan SzetoJie Runext slide

Au strip

500 µm

1 mm

Junction area:

2.5 x 2.5 µm to400 x 400 µm

V

Vsense

+-

current amplifier

Cross section of a PPF/NAB/Cu/Au junction (SEM)

Molecules (not resolved)

Si

PPF

SiO2

SiNCu/Au

Left side Right side

Si

SiO2

PPFMolecules (not resolved)

Cu/Au SiN

SiO2

Si

PPFmolecules

Cu/AuSiN SiNSiO2 SiO2

Schematic of junction structure:

molecular layer is really thin compared to metals,does it survive metal deposition??

Cu/Au

PPF

SiN/SiO2

to scale:

1-5 nm

> 100 nm

Au 30nm

Cu 120nmSiN

70nmSiO2 50nm

PPF 1µm

(not to scale)

~4 nm NAB

~10 nm PPF

Quartz substrate(0.13 mm)

514.5 nm laser

excitation at

45°Collect Raman

scattered light

normal to

substrate

Au

CuNO2

N=N

(~ 4 nm thickmultilayer)

“backside”

Raman of buried interface:

~50% transparent

Quartz/PPF/NAB/Cu/Au(after metal deposition)

Quartz/PPF/NAB, no

Cu or Au

3000

5000

7000

9000

11000

13000

15000

17000

600 1100 1600

Intensity (counts/30 s)

Raman Shift (cm-1)

Ram

an in

tens

ity (3

0 se

c, 1

9 m

W)

NO2

N=N

Adam BergrenAmr

Mahmoud

(Monday, 4:20 PM)

NAB on Au/Ti

Au/Ti NAB

AuTi

-log

(R/R

o)

wavenumber, cm-1

FTIR of buried interface:

Ti is “primer layer”

forNAB bonding to Au

wavenumber, cm-1

-log

(R/R

o) Au

IR transparent Si

IR beam

after 100 nm Au deposition:

Adam BergrenAmr

Mahmoud

(Monday, 4:20 PM)

carbon/molecule

V

Vsense

+-

current amplifier

Cu

Au

= fluorene

carbon on SiO2

12 nm

20 nm

1.7 -

2 nm

2 μm

V

Electronic behavior:

Labview

-25

-20

-15

-10

-5

0

5

10

15

20

-0.2 -0.1 0 0.1 0.2

ControlFL-SiO2

PPF/SiO2

/Cu

PPF/Cu slope= 1.6 Ω

curr

ent d

ensi

ty, J

, A/c

m2 V

SiO2

Cu

10 nm

V, carbon relative to Au

-0.002

-0.0015

-0.001

-0.0005

0

0.0005

0.001

0.0015

0.002

-4 -3 -2 -1 0 1 2 3 4

J, A

/cm

breakdown~ 3 MV/cm

slope= 420,000 Ω

Start with something familiar:

VPPF

Cu

SiO2

Au

-3

-2

-1

0

1

2

3

-2.4 -1.8 -1.2 -0.6 0 0.6 1.2 1.8 2.4

NBP film muchmore conducting than SiO2

How about a molecule instead of SiO2

?

NO2

nitrobiphenyl multilayer4.6 nm thick:

Cu

NO2

J, A

/cm

2

PPF

4.6 nm

V, carbon relative to Au or Cu

NO2

-3

-2

-1

0

1

2

3

-2.4 -1.8 -1.2 -0.6 0 0.6 1.2 1.8 2.4

V

J, A

/cm

2

NBP4.5

FL-SiO2

NBP 2.8 nm

NBP 1.6

Nitrobiphenyl junctions of differing thickness: V

NBP

Au

4.6 nm2.8 nm1.6 nm

SiO2rsd

5-15%yield > 90%

typical errorbar

7 A/cm2

~ 106

e-/sec/molecule

J. Phys. Chem. B, 2005, 109, 11163

• No obvious shape change from 1.6 to 4.6 nm thickness

• Symmetric with minimal hysteresis

• Repeatable > 108

cycles

-6

-5

-4

-3

-2

-1

0

1

2

3

-1 .5 -0 .5 0 .5 1 .5

V , V

ln J

, A/c

m2

V

Same data, on a log scale:NBP(1.6 nm)

NBP(2.8 nm)

NBP(4.6 nm)

ln(J, 0.1 V)

-16

-13

-10

-7

Ln I

(V=0

.1 V

), A

1 2 3 4 5d= thickness, nm

β

= 0.22 Å-1

NO2

β

= 0.21 Å-1

J = B e-βd

Literature: β• alkanes

(echem

or junctions) 0.8 Å-1

• aromatic (echem, 1999) 0.22• conjugated (echem, SAM)

0.3 to 0.6

• polyene* (2005) 0.22• oligothiophene* (2008) 0.1• oligoporphyrin* (2008) 0.04• oligophenyleneimine+

(2008) 0.3

* single molecule junction+ ~100 molecule junction

d

β

is smaller for aromatic structures(i.e. conjugated molecules are

better “conductors”)

‐15‐10‐5051015202530

‐1 ‐0.5 0 0.5 1V (PPF relative to Au)

j(A cm

‐2)

no molecule

(2.0 nm)(1.4 nm)

(1.5 nm)

NO2

(3.7 nm multilayer)

(1.9 nmmultilayer)

NO2

strong effectof structure and

thickness on conduction

Is thisbehaviormolecular?

Adam Bergren, J Phys. Cond. Matt. 2008, 20, 374117

Various transport mechanisms:

Field emission (Fowler Nordheim)

Incoherent, diffusive tunneling

Coherent tunneling, "superexchange"

Weakly Temperature dependent: distance dependence:

exp(-

β

d)

exp(-

β’d)

(V/d) exp(-

a d)

“hopping”, including redox exchange(Marcus-Levich)

Poole-Frenkel effect (“coulombic traps")

Thermionic (Schottky) emissionStrongly Temperature dependent (“activated”):

d-1

exp (-c d1/2)

exp (-c’d1/2)

400 K 325 K250 K

100,120,150 K

-0.2 0.0 0.2 0.4V

-3.0

-2.0

-1.0

0.0

1.0

2.0

3.0

-0.4

J, A

/cm

2

j (0.2 V)

J. Phys. Cond. Matter, 20, 374117 (2008)

“activated”

temperatureindependent

A good probe of mechanism:Temperature dependence

‐4.5

‐4

‐3.5

‐3

‐2.5

‐2

‐1.5

‐1

‐0.5

0

0 50 100 150 200 250

ln j 0

.2 V (A

cm

‐2)

1000 T‐1 (K‐1)

BP(1.4) FL(1.8) NAB(3.3) AB(3.2)

5 K10 K20 K40 K

Arrhenius plotsArrhenius plots

AJB 25

(1.8 nm)

(1.4)

NO2

N=N

(3.3 nm)

‐4.5‐4

‐3.5‐3

‐2.5‐2

‐1.5‐1

‐0.50

0 5 10 15 20 25

1000 T ‐1 (K‐1)

100 K200 K

26 µeV

56 µeV

50 µeVNO2

N=N

46 meV

31

137(AB)

40 meV(NAB)

‐10

‐5

0

5

10

15

‐1.5 ‐1 ‐0.5 0 0.5 1 1.5

V

j (A/cm

2 )

(4.5 nm multilayer)

NO2

N=N

T = 5 K

‐0.01

‐0.005

0

0.005

0.01

‐0.1 ‐0.05 0 0.05 0.1

V

j (A/cm

2 )

need to explain:

• apparent ohmic

contact• T-independent, despite:• 4.5 nm thick (too thick

for tunneling)

(T independent, 5 –

250 K)

NAB experimental (2.6 nm)

-100-80

-60-40-20

0

2040

6080

100

-1 -0.5 0 0.5 1

Applied Voltage (V)

J (A

/cm

2) PPF/NAB (2.6nm )/Cu

Simmons withimage charge

Simmons, φ

= 0.85 eV(i.e. tunneling through arectangular barrier)

Field emission (Fowler Nordheim)

All common off-resonance tunneling mechanisms fail:

-2

-7

-6

-4

-3

-5

Simmons, J. G. Journal of Applied Physics (1963), 34, 1793‐1803

m* = effective electron mass, where mass of charge carrier = m* x 9.1 x 10‐31

kg

Experimental data collected at 5 or 6 K

( )⎪⎭

⎪⎬⎫

⎪⎩

⎪⎨⎧

⎟⎟⎠

⎞⎜⎜⎝

⎛⎟⎠⎞

⎜⎝⎛ +−×⎟

⎠⎞

⎜⎝⎛ +−

⎥⎥⎦

⎤

⎢⎢⎣

⎡⎟⎠⎞

⎜⎝⎛ −−×⎟

⎠⎞

⎜⎝⎛ −= dqVmqVdqVmqV

dqJ

2*22exp

22*22exp

24 22 φφφφπ ηηη

Tunneling with reduced electron mass (modified Simmons equation):

Φ

= 1.1 eVm* = 0.4 me

‐3

‐2

‐1

0

1

2

3

4

‐1.5 ‐1 ‐0.5 0 0.5 1 1.5

NAB(3.3), experiment

Simmons Modeld= 3.3 nm

“off resonant”

tunneling just doesn’t work

‐0.3

‐0.2

‐0.1

0

0.1

0.2

0.3

0.4

‐1.5 ‐1 ‐0.5 0 0.5 1 1.5

NAB(4.5), experiment

Simmons Model

Φ

= 1.1 eVm* = 0.4 me

d= 4.5 nm

*experimental, from Kelvin probe

-2

-6

-4

-3

-5

-7

NAB LUMO (-3.0 eV

, from DFT)

HOMO (-6.6 , DFT)

Cu (-4.7)*PPF(-4.9)*

eVvs

vacu

um

we expect these levels tobe broadened, by:

• electronic coupling to substrate• intermolecular interactions• variable bonding geometry• uncertainty (i.e. lifetime) broadening

NO2

N=N

0.02

0.04

0.06

200 300 400 500 600 700

NAB in hexane (X.02)

wavelength, nm

Opt

ical

Abs

orba

nce

Some evidence for broadening:

NAB bonded to carbon

Appl. Spectros. 2007, 61, 1246-1253

NO2

N=N

An alternative approach:

NAB

-6

-4

-2

E, eV

DFT with periodic boundary conditions for graphene

Stan StoyanovKirill

KoshelevAndriy

KovalenkoNINT

HOMO

LUMO

NAB-graphene

HOMO and LUMO energies vary with torsion angle

Modeling of both contacts and molecule

Koshelev

-6

-4

-2

C_NAB_Au

E , eV

HOMO

LUMO

C_NAB_Cu strong interactionof both Cu andgraphene

with NAB

ener

gy re

lativ

e to

vac

uum

, eV

a range of orbital energies

-2

-7

-6

-4

-3

-5

distance

LUMOs

HOMOs

filled statesin metal

Efermi

Zero bias: positive bias:

+-

next slide

V=0

Note that more HOMOsbecome accessible for

higher bias, causing upwardcurvature

e-

HOMO fills again fromnegative electrode,

effectively “hole transport”

HOMOs

once + bias createsempty metal orbitals,electrons can leave

HOMO

e-

+ bias

+

-

2.6 nm

more HOMOs

becomingaccessible with increased bias

NO2

N=N

-70

-50

-30

-10

10

30

50

70

90

-1 -0.5 0 0.5 1Applied Voltage (V)

J (A

/cm

2)

observedgaussian

distributionσ

= 0.52 eVEf

– EHOMO

= 1.7 eVNchan

= 105

sech

distributionσ

= 0.31 eVEf

– EHOMO

= 1.7 eVNchan

= 105

σ

= half width of orbital energy distributionEf

– EHOMO

= Fermi level to orbital offsetNchan

= total number of active channels

Sergio JimenezAdam Bergren

PPF/NAB (2.6 nm)/Cu

-5-4-3-2-1012345

-1 -0.5 0 0.5 1

Applied Voltage (V)

Ln (J

)

NO2

N=N

• Ef

– EHOMO• HOMO “linewidth”

(σ)• and number of channels (N)• molecular layer thickness (d)

Main parameters of the model:

sech

observed

gaussian

for the electrochemists:NOT Marcus/Butler-Volmer; similarity due to distributionof orbital energies ratherthan thermal fluctuations

Important notes:

HOMO

LUMO

Ef

ener

gy

σ

EF

- EHOMO

• broadening caused by coupling and localenvironment, not thermal fluctuations

• main parameters are distribution width (σ),energy offset (EF

– EHOMO

), and thickness

• overlap of metal and molecule orbitals may(and probably does) occur at zero bias

• depending on offset between molecular orbitals andFermi level, we can greatly vary conductance

The punch line: strong interactions between molecule and contactsresult in resonant

electron transport rather than classical tunneling

Adam Bergren (NRC)Sergio Jimenez (visit. prof.)Andriy

KovalenkoStan StoyanovKirill

KoshelevJie Ru (Uof

Alberta)Bryan Szeto

Also :

Rory Chisholm (2:00 PM Monday)Mark McDermott

NINT