scanned probe imaging of switching centers in molecular devices hp labs quantum science research...
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Scanned Probe Imaging of Switching Centers in Molecular Devices
HP Labs
Quantum Science Research
Chun Ning (Jeanie) Lau
Dr. Duncan Stewart
Dr. R. Stanley Williams
Prof. Marc Bockrath (Caltech)
Molecular Electronics
Challenges
•Fabrication
•Architecture
•New devices
•Ultimate limit of miniaturization
•Self-assembly low fabrication cost
Fault-tolerant Architecture
HPL TeraMAC1 THz multi-architecture computer• Largest defect-tolerant
computer• 220,000 (3%) defective
components• 106 gates operating
at 106 cycle/sec• addresses problem of
<100% yield
Heath et al, Science (1997).
Nano-imprint Lithography
• fast fabrication of nm scale features over cm area
Substrate
MoldMold
SubstrateSubstrate
Y. Chen, G.Y. Jung et al. (2003).6 Gbits/cm2
Unsolved Question
What are the switching mechanism(s)?
Proposed:• conformational change of molecules• eletrical charge transfer, electron localization• molecule-metal contacts
Molecular Switches
• Previously studied systems: Nanopore, STM, cross-bar
Molecules studied: rotaxane, catanane, OPE, etc
• Potential applications as memory or logic devices
Switching of Metal/Alkane/Metal Junctions
Langmuir-Blodgett films of molecular monolayer sandwiched between m-sized metallic electrodes
V
A
TiPt
Our Experiment
COH
OH3C
C18H36O2
Stearic acid:
• electrical insulator HOMO-LUMO gap ~ 8eV
• no redox centers, mobile subgroups, or charge reception sites
2.6 nm
Switching of Metal/Alkane/Metal Junctions
• Asymmetric electrodes (Ti & Pt)
• Reversible switching dependent on bias direction
-1000
-500
0
500
1000
Cu
rren
t A
)
1.00.50.0-0.5Voltage (V)
1 2
3
4
V
A
Ti
Pt
Stewart et al, Nano Lett., in press.
Novel Scanned-Probe Technique
• Apply ~N force with AFM tip while measuring device conductance
• Simultaneously explores electrical and local mechanical properties
• AFM tip not electrically connected to the device
Si Substrate
Pt
Ti
AFM
• Plot conductance through junction as a function of tip position
• AFM tip applies ~N force pressure ~ 103 – 104 atm
AFM Imaging of Mechanically-induced Conductance Response
Conductance Map (“off” state)
Topography
Conductance (Off)
15 m
A
V
Molecular junction in the “off” state exhibited no observable electrical response to local mechanical perturbation by the AFM tip.
1.0
0.5
0.0
I(
0.50.0V(V)
off
1.0
0.5
0.0
I(
0.50.0V(V)
off
on
Conductance Map (“On” state)
Topography
Conductance (Off)
15 m
Conductance (On)
• A nanoscale conductance peak (“switching center”) emerges when the junction turn “on”.
1.0
0.5
0.0
I(
0.50.0V(V)
off
on
-20
-10
0
10
I (n
A)
-0.5 0.5Vbias
Off
400
200
0
-200
-400
I
-0.4 -0.2 0.0 0.2 0.4
Vbias
on
Off
-1.5
-1.0
-0.5
0.0
0.5
1.0
I
-0.4 -0.2 0.0 0.2 0.4
Vbias
Off
“Switching Centers”
on
Switching “on” of a device is always accompanied by the emergence of a new nanoscale pressure-induced conductance peak.
50 nm
1.6
1.5
1.4
1.3
1.2I
1086420
Tip Position (m)
0.0500.045
Switching “off”
300
200
100
0
I
43210Vbias
The switching center faded and completely vanished with successive switchings to lower conductance states.
1
2
3
Our Experimental Finding
Under mechanical pressure, a single nanoscale conductance peak (switching center) appears when the junction is switched “on”, and disappeared when “off”.
Formation and dissolution of nanoscale structural inhomogenities on the junction give rise to switching.
Lau et al, in preparation.
What are these inhomogenities?
A Simple Model
• Transport across molecular monolayer via tunneling• When switched “on”, electrodes move closer together
within a nanoscale region dominate transport• Conductance only increase when the AFM tip is
compressing the nano-asperity.
Nano-asperity
(top or bottom electrodes)
Applying Pressure with AFM tip
• Monolayer compressed by z (~ 0.2 Å for F ~ 1 N)• Spatial resolution ~ 40 nm : Limited by tip radius and thickness of top electrode.
Elasticity theory (Landau&Lifshitz)
point force applied to semi-infinite plane strain at (x,y,d)
F
z=0
d F
dyx
d
Euzz 2/5222
3
2
3~
E = Young’s modulus ~ 80 GPa for metals and alkane moleculesd = thickness of top electrode ~ 30 nm
A Simple Model• Nano-asperity dominate transport
Model
No free parameters : Goff~ 0.1S, Gon~1.3S, ~1Å-1, z (0,0) ~ 0.2 Å
• switching “on” ↔ growth of asperity
Data
0 500
0.0
127.5
col
row
-0.1625 0.2000
ana15_05fa_short3D_sm
1.2 S 1.4
G = Goff + (Gon-Goff )exp (- z(x, y))
nano-asperity located at (0,0)“off” state conductance
• Good agreement between model and data
Nanoscale Filaments
-20
-10
0
10
I (n
A)
-0.5 0.5Vbias
Off
400
200
0
-200
-400
I
-0.4 -0.2 0.0 0.2 0.4Vbias
on
Off
• G>>conductance quantum GQ Continuous filamentary pathway
• switching ↔ formation and dissolution of nano-filaments
• Nature and growth mechanism of filaments?(thermal migration, electrochemical reaction, electro-migration…)
0 500
0.0
127.5
col
row
-0.1625 0.2000
ana15_05fa_short3D_sm
850 860S
On/off ratio ~ 105
Effect of Force on Conductance
Conductance within a “switching center” increases with increasing applied force.
nm
Current
Vbias = 0.1 V
Force
0.1 N
0.6 N
1.5 N
3 N
tunnel barrier
Conclusion
• Novel experimental technique that probes nanocale conductance pathways through molecular junctions
• New switching mechanism with high on/off ratio due to formation and break-down of conductive nano-filaments
Under Investigation
• filament growth mechanism
• pressure dependence
• other systems (other molecules, different electrodes)
• role of electrodes in metal/molecule/metal structures better engineering of molecule-based devices.
We’re only at the base camp….
•molecular devices
•superconducting nanowires & nanorings
•ferromagnetic nanowires
•single molecules
•carbon nanotubes
•nanosensors
……
mastery of nanoscale systems
Silicon Electronics
Wires & Switches
ArchitectureLithography & Deposition
Complex physical structurePerfect fabrication
Memory & Logic
Nanoelectronics
Wires & Switches
Chemical synthesis & assembly
Simple physical structureImperfect fabrication
Architecture?
Memory & Logic
Nanoscale filaments
Nanoscale filaments grows or shrinks switching
•Electromigration?
•Elecrochemical migration?
•Thermal migration?
•Single dominant pathway Runaway process?
-20
-10
0
10
I (n
A)
-0.5 0.5Vbias
Off
400
200
0
-200
-400
I
-0.4 -0.2 0.0 0.2 0.4
Vbias
on
Off
-1.5
-1.0
-0.5
0.0
0.5
1.0
I
-0.4 -0.2 0.0 0.2 0.4
Vbias
Offon
“Switching Centers”
Switching “on” of a device is always accompanied by the emergence of a new nanoscale pressure-induced conductance peak.
Device A
Nanoscale Filaments?
860
850G
S)
2500Tip Position (nm)
• Unperturbed conductance ~ 800 S ~ 10 GQ
• Small increase in conductance under pressure ~1%
Device B Device A
1.3
1.2
G
S)
2500Tip Position (nm)
• Unperturbed conductance << GQ
• Relatively large increase in conductance under pressure ~16%
Nano-imprint Lithography
Substrate
MoldMold
SubstrateSubstrate
Y. Chen, G.Y. Jung et al.
• nm scale features over cm area
6Gbits/cm2
Fault-tolerant Architecture
HPL TeraMAC1 THz multi-architecture computer• Largest defect-tolerant
computer• 220,000 (3%) defective
components• 106 gates operating
at 106 cycle/sec• Built from programmable gate
arrays• Computes with look-up tables
Collier et al, Science 1998.
Philip Kuekes
Nano-imprint Lithography
Substrate
MoldMold
SubstrateSubstrate
Y. Chen, G.Y. Jung et al.
• fast fabrication of nnm scale features over cm area
6Gbits/cm2
Topography
Conductance image
Nano-conducting Channels
Bottom Electrode
Top Electrode
0.0 0.5 1.0 1.5 2.0 2.5
18.5
19.0
0 2
Tip position (m)
18.0
R (
k
19.0
A local dominant nanoscale conducting channel
Switching On of Molecular Junctions
0.14
0.13
I
1.00.50.0Tip position (m)
A conductance hot-spot appeared under mechanical modulation when the junction was switched on.
• Diameter ~ 50 nm ~ AFM tip radius
• 10% increase in conductance under ~2 N
400
200
0
-200
-400
I
-0.4 -0.2 0.0 0.2 0.4Vbias
Another Device
A new hot spot always appeared after switching “on” the junction
1.025 1.112 1.200Current (nA)
-20
-10
0
10
I (nA
)
-0.5 0.5Vbias
86.0
85.0
I
5002500
Tip Position (nm)
Topography Conductance
85.0 86.0current (A)
Off
On
Off
On
Transport Through Molecular devices
100 nm
Single Molecule Measurements Nanoscale junctions
2m
Pt
Ti
-1.2
-0.9
-0.6
-0.3
0
-2 -1 0 1 2
Cu
rren
t (m
A)
Voltage (V)
0
5
10
15
20
0 0.5 1 1.5 2 2.5
Cu
rre
nt
(nA
)
Voltage (V)
Ti
S
O
CH3
Pt
13
3
V
Chang et al, APL (2003)
T(K)
C18H36OH
Pt
Pt V
Stewart et al, in preparation.diode r~5 x 105
Molecular Electronics
•Ultimate limit of miniaturization
•Self-assembly low fabrication cost
•“Designer molecules”