Stuart.Lindsay@asu.edu
Arizona Biodesign Institute
Single molecules as a tool for understanding
More Single molecules for better understanding

Photosynthesis and single molecule electronics
The ASU-Physics-Chemistry-Engineering-Motorola group:

Measure Single Molecule in well-defined conditions
Compare with theory (repeat as needed!)
‘Calibrated’ molecules and contacts for devices
Make fixed-gap (useful? T-dep?) devices

The molecule-metal contact problem
Many Few-Molecule-Devices have been made but measurements/theories generally do not agree:
For example, DNA is:
AN INSULATOR
(D. Dunlap et al. PNAS 90, 7652, 1993)
A SEMICONDUCTOR
(D. Porath et al, Nature 403,
635, 2000)
A CONDUCTOR
(Fink and Schoenberger, Nature 398,
407,1999)
A SUPERCONDUCTOR
(A.Y. Kasumov et al. Science
291, 280, 2001)

1. Self-assembly: well defined geometry, contacts
2. Repeated contact (simpler after answer is known)
3. Fixed nano-gaps: problem of manufacturability

Self-assembled nanojunction
(Science 294 , 571, 2001) i

Alkanedithiols – STM Images

IV-Curves are integral multiples
10
0
-10
-20
40
30
20
-30
-40
-1.0
1I(V)
2I(V)
3I(V)
4I(V)
5I(V)
-0.5
0.0
Tip bias (V)
0.5
1.0
8
6
4
2
0
-2 1I(V)/1
-4
2I(V)/2
3I(V)/3
-6
4I(V)/4
5I(V)/5
-8
-1.0
-0.5
0.0
0.5
1.0
Tip bias (V)

Two Models for ‘quantized ‘ data

Histogram of curve multipliers
700
600
500
400
300
200
100
0
0 1 2 3 4 5
Current divisor X
• Find X such that variance from curve to curve is minimized
• Over 1000 curves for n=1

IV-Curves of bonded molecules not very stress dependent!
Current not stressdependent – through bond?
Bonded
10
8
6
F
4
I

F(h)
2
0
Mechanical
-2
-24 -20 -16 -12 -8 -4 0 4
Distance moved (nm)
(Nanotechnology 13 5-14, 2002)
100
10
1
0.1
0.01
0.001
0.0001
10
-5

I is closer to theory for bonded molecules
Theory
Bonded
100
10
1
0.1
0.01
R(C
8
)=950M

0.001
0.0001
0 0.2
0.4
0.6
0.8
Tip Bias (V)
Mechanical
1
Comparison with electrochemistry:
R
 k
B
T e
2
K ( E
A

0 )
K ( E
A

0 )
 exp




K
E
A k
B
T 


K =10 5 s
R(C
-1
8
, E
A
=21kJ/m,
)=300M


More chain lengths give

(V)
(J. Phys. Chem. B 106 8609-
8614, 2002 )
I=I
0 exp[-

(v)z]
• Also measure
Ohmic region carefully to get

(0)
• Data do NOT agree with theory!
15
10
50
5
0 0
-5
-10
(CH
2
)10 0.04
-15
-1 -0.5
0 0.5
1
Tip Bias (V)
0
-0.04
1 2 3 4
4
0
-1
3
2
1
-0.05
30
0
-2
)12
0.02
-3
(CH
2
-4
-1 -0.5
0 0.5
1
Tip Bias (V)
0
-0.02
0
1 2 3 4
0.05
-0.05
0 0.05

8
6
4
2
0
-2
-4
-6
-8
-1 -0.5
0 0.5
1
Tip Bias (V)
What if the top contact is not so good? Coulomb Blockade?

R
1
>>h/2e 2

Coulomb
Blockade n n

Coulomb Blockade fits
(Removes Beta anomaly)
12
8
4
0
-4
-8
C8
-12
-2 -1.5
-1 -0.5
0 0.5
1 1.5
2
Tip Bias (V) r
1
= 1M
 c
1
= 0.318aF
(from fitting C8)
2
6
4
C10
0
-2
-4
-6
-2 -1.5
-1 -0.5
0 0.5
1 1.5
2
Tip Bias (V)
-0.5
-1
-1.5
2
1.5
1
0.5
0
C12
-2
-2 -1.5
-1 -0.5
0 0.5
1 1.5
2
Tip Bias (V) r
8
= 128.36M
 r
10
= 252M
 r
12
= 875M

(c
1
, r
1 fixed)
C8=c10 =c12= 0.085aF
(Theory=0.08aF)
Good agreement for I,


- Carotene
- Phenylene-ethylenine oligomers
Technique reveals mobility of Au contacts

HS
Caroteniod
(J. Phys. Chem. B, 107, 6162-6169, 2003)
SH
32 Å
TRANS
T/C=4:1
CIS t

Trans
Simple Tunneling with
Coulomb Blockade fits well
No ‘free’ parameters
10 3
5 a
Measured
2
1 b
0
0.2
-5 0
-0.2
-0.2
0 0.2
-10
-1.5
-1 -0.5
0 0.5
1 1.5
Tip Bias (V)
0
-1
R1 R2
-2
C1 C2
-3
-1.5
-1 -0.5
0 0.5
1 1.5
Tip Bias (V)
Cis
Carotene easily oxidized but vibronic contribution not dominant

Phenylene-ethylynene Oligomers and Negative Differential
Resistance
NO
2
AcS SAc
1
AcS SAc
2
1 = 1-nitro-2,5-di(phenylethynyl-4´-thioacetyl)benzene
2 = 2,5-di(phenylethynyl-4´-thioacetyl)benzene
Applied Physics Letters 81 3043-3045 (2002)

Single Molecule NDR
1.5
1
0.5
0
1.8G

-0.5
-1
50G

-1.5
-1.5 -1 -0.5 0 0.5
1 1.5
Tip Bias (V)
1 12
10
8
6
4
2
0
0 0.5
1 1.5
2
Peak Volts 2
Asymmetry, but molecules
NOT oriented?
c.f. Reichert et al. PRL 88 2002
Confirms NDR but see nonreversible behavior

Single Molecule Bonding
Fluctuations
(Science 300 1413, 2002)
• “Stochastic switching” reported
NO
2 for
• We see the same effect in alkane dithiols
• Significant switching with gold sphere attached

Single Molecule Bonding
Fluctuations – Property of Au-S
• Cannot internal electronic changes
• Cannot be top ‘dipping’ into film
• Cannot be bond to sphere breaking
• Rate increases at annealing temperature
• Fluctuations of lower bond
25

60


Transport in goldnalkane-gold fits ab-initio tunneling calculations well: One molecule, well defined environment.
Gold nanocluster introduces Coulomb Blockade
Carotene – good agreement with tunneling calculations
Phenylene-ethylenine oligomers, confirm NDR.
Technique reveals mobility of Au contacts

Break Junctions
- Geometry unknown BUT
- Calibration available AND
- Most common peak appears to be correct
Much easier than self-assembled junctions
(Xu and Tao, Science 301 , 1221-1223 2003)

Single Molecule Conductance from
Break Junctions
PZT
1
2
• Are histogram peaks good data points?
• What is the effect of strain?
3

Gold filaments stretch – maximum force ca 1nN
Xu, Xiao, Tao,
JACS 2003

Alkanedithiols:
SH
N=6:
HS
R
C6
=10.5 M

N=8: HS
R
C8
=51 M

SH
SH N=10: HS
R
C10
=630 M

Major peaks are right peaks

a b
Fixed Gaps:
EBL, Electrochemistry, Electromigration
Au c
SiO
2
200nm d
Au
1um
200nm e
Kubatkin et al. (2003) –
( but 3 devices/paper )
500nm
2um

Our Fixed Gaps: 10% “Success
Rate”?
Molecule free molecular transistor
77K
90
80
70
60
50
40
30
20
10
0
-10
-20
Vg=-0.5V
Vg=-0.25V
Vg=0V
Vg=0.25V
Vg=0.5V
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
SDbias(V)
Atomic-scale control needed!

1. Measurements and theory in good agreement for some (single) molecules
(n-alkanes, carotenes, BDT
[Tao, nanolets 4 267]
)
2. Break junctions can give good data and are much simpler

1. Single Molecule devices need to be assembled with Atomic Precision!
2. Fluctuations at room temperature?
3. Couplings and molecular vs. contact properties?
4. Redox activity molecules and environment?

ACKNOWLEGEMENTS
ASU PHYSICS
Xiadong Cui
Otto Sankey
John Tomfohr
Jun Li
Jin He
ASU Engineering
Nongjian Tao
ASU Chemistry
Ana Moore
Tom Moore
Devens Gust
Xristo Zarate
Alex Primak
Yuichi Terazano
Motorola
Gary Harris
Larry Nagahara