Transcript Single Molecule

Single Molecule Electronics
[email protected]
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:
The Program
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)
Making Single Molecule
Junctions
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
40
1I(V)
30
Current (nA)
20
NI(V )/N (nA)
2I(V)
3I(V)
4I(V)
10
5I(V)
0
-10
-20
-30
-40
-1.0
-0.5
0.0
Tip bias (V)
0.5
1.0
8
6
4
2
0
1I(V)/1
-2
2I(V)/2
-4
3I(V)/3
4I(V)/4
-6
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!
Bonded
100
8
10
Force (nN)
10
F
IF(h)
1
6
0.1
4
0.01
2
0.001
0
0.0001
Mechanical
-2
-5
-24 -20 -16 -12 -8 -4 0
4
Distance moved (nm)
(Nanotechnology 13 5-14, 2002)
Current (nA)
Current not stressdependent –
through bond?
10
I is closer to theory for bonded
molecules
Theory
Bonded
Current (nA)
100
10
k BT
R 2
e K ( E A  0)
1
0.1
R(C8)=950M
0.01
K ( E A  0) 
0.001
0.0001
Comparison with
electrochemistry:
0
0.2
0.4
0.6
Tip Bias (V)
Mechanical
0.8
1
K
 EA 
exp

k
T
 B 
K=105s-1, EA=21kJ/m,
R(C8)=300M
I=I0exp[-(v)z]
• Also measure
Ohmic region
carefully to get
(0)
• Data do NOT
agree with theory!
Current (nA)
(J. Phys. Chem. B 106 86098614, 2002 )
15
50
10
5
0
-5
-10
0
(CH2)10
-15
-1 -0.5 0 0.5
Tip Bias (V)
Current (nA)
More chain
lengths give
(V)
4
3
2
1
0
-1
-2
-3
(CH2)12
-4
-1 -0.5 0 0.5
Tip Bias (V)
1 2 3 4
0.04
0
1
-0.04
-0.05
30
0
0
0.05
1 2 3 4
0.02
0
1
-0.02
-0.05
0
0.05
Clue: I-V doesn’t fit
tunneling model well
8
6
Current (nA)
4
2
0
-2
-4
-6
-8
-1
-0.5
0
Tip Bias (V)
0.5
1
What if the top contact is not so
good? Coulomb Blockade?
Coulomb Blockade:
Quantized charge transfer
R1>>h/2e2
 Coulomb
Blockade
n
n
Coulomb Blockade fits
(Removes Beta anomaly)
2
12
6
C8
4
0
-4
2
0
-2
-8
-4
-12
-6
-2
-1.5
-1
-0.5
0
0.5
1
1.5
2
Current (nA)
4
Current (nA)
Current (nA)
8
1.5
C10
1
C12
0.5
0
-0.5
-1
-1.5
-2
-1.5
-1
Tip Bias (V)
-0.5
0
0.5
1
1.5
2
-2
-2
-1.5
Tip Bias (V)
-1
-0.5
0
r1 = 1M
r8 = 128.36M
C8=c10 =c12= 0.085aF
c1 = 0.318aF
r10 = 252M
(Theory=0.08aF)
(from fitting C8)
r12 = 875M
(c1, r1 fixed)
0.5
1
Tip Bias (V)
Good agreement for I, 
1.5
2
Other Measurements
- Carotene
- Phenylene-ethylenine oligomers
-Technique
contacts
reveals mobility of Au
Caroteniod
(J. Phys. Chem. B, 107, 6162-6169, 2003)
SH
HS
32 Å
TRANS
T/C=4:1
CIS
t
Simple Tunneling with
Coulomb Blockade fits well
No ‘free’ parameters
5
3
aMeasured
0
2
Current (nA)
Current (nA)
10
0.2
0
-5
b
1
0
-1
R1
R2
C1
C2
-2
Trans
-10
-1.5
-0.2
-0.2
-1
-0.5 0
0.5
Tip Bias (V)
0
0.2
1
1.5
-3
-1.5
-1
-0.5 0
0.5
Tip Bias (V)
1
Cis
Carotene easily oxidized but
vibronic contribution not dominant
1.5
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
1.8G
Count
Current (nA)
1
1
0.5
0
-0.5
2
50G
-1
-1.5
-1.5 -1 -0.5 0 0.5
Tip Bias (V)
12
10
8
6
4
2
0
0
0.5 1 1.5
Peak Volts
2
Asymmetry, but molecules
NOT oriented?
1
1.5
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
for
NO2
• 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
Conclusions
Transport in gold-n-alkane-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:
N=6:
HS
SH
RC6=10.5 M
N=8:
HS
SH
RC8=51 M
N=10:
HS
SH
RC10=630 M
Major peaks are right peaks
a
b
Fixed Gaps:
EBL, Electrochemistry, Electromigration
c
200nm
SiO2
Au
d
200nm
Kubatkin et al. (2003) –
(but 3 devices/paper)
e
Au
500nm
1um
2um
Our Fixed Gaps: 10% “Success
Rate”?
Molecule free molecular transistor
77K
Current(uA)
90
80
70
60
50
40
30
20
10
0
-10
-20
-1.5
Vg=-0.5V
Vg=-0.25V
Vg=0V
Vg=0.25V
Vg=0.5V
-1.0
-0.5
0.0
0.5
1.0
1.5
SDbias(V)
Atomic-scale control needed!
Summary 1:
What we think we know
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
Summary 2:
What we don’t know/Can’t do
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