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 TeraMAC 1 THz multi-architecture computer Largest defect-tolerant computer 220,000 (3%) defective components 10 6 gates operating at 10 6 cycle/sec addresses problem of <100% yield Heath et al, Science (1997). Nano-imprint Lithography fast fabrication of nm scale features over cm area Y. Chen, G.Y. Jung et al. (2003). 6 Gbits/cm 2

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 Ti Pt Our Experiment C OH O H 3 C C 18 H 36 O 2 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 Current  A) Voltage (V) 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 ~ 10 3 – 10 4 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. 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”. off on

Off I  Vbias on Off I  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

I  Tip Position (  m) Switching “off” I  Vbias    The switching center faded and completely vanished with successive switchings to lower conductance states

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 E = Young’s modulus ~ 80 GPa for metals and alkane molecules d = thickness of top electrode ~ 30 nm

A Simple Model Nano-asperity dominate transport Model No free parameters : G off ~ 0.1  S, G on ~1.3  S,  ~1Å -1,  z (0,0) ~ 0.2 Å switching “on” ↔ growth of asperity Data 1.2  S 1.4 G = G off + (G on- G off )exp (-  z(x, y)) nano-asperity located at (0,0) “off” state conductance Good agreement between model and data

Nanoscale Filaments Off I  Vbias on Off G>>conductance quantum G Q  Continuous filamentary pathway switching ↔ formation and dissolution of nano-filaments Nature and growth mechanism of filaments? (thermal migration, electrochemical reaction, electro-migration…)  S On/off ratio ~ 10 5

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 Architecture Lithography & Deposition Complex physical structure Perfect fabrication Memory & Logic Nanoelectronics Wires & Switches Chemical synthesis & assembly Simple physical structure Imperfect fabrication Architecture? Memory & Logic

Nanoscale filaments Nanoscale filaments grows or shrinks  switching Electromigration? Elecrochemical migration? Thermal migration? Single dominant pathway  Runaway process?

Off I  Vbias on Off I  Vbias Off on “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? Unperturbed conductance ~ 800  S ~ 10 G Q Small increase in conductance under pressure ~1% Device B Device A G  S) 2500 Tip Position (nm) Unperturbed conductance << G Q Relatively large increase in conductance under pressure ~16%

Nano-imprint Lithography Y. Chen, G.Y. Jung et al. nm scale features over cm area 6Gbits/cm 2

Fault-tolerant Architecture HPL TeraMAC 1 THz multi-architecture computer Largest defect-tolerant computer 220,000 (3%) defective components 10 6 gates operating at 10 6 cycle/sec Built from programmable gate arrays Computes with look-up tables Collier et al, Science Philip Kuekes

Nano-imprint Lithography Y. Chen, G.Y. Jung et al. fast fabrication of nnm scale features over cm area 6Gbits/cm 2

Topography Conductance image Nano-conducting Channels Bottom Electrode Top Electrode 02 Tip position (  m) 18.0 R (k  19.0 A local dominant nanoscale conducting channel

Switching On of Molecular Junctions 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

Another Device A new hot spot always appeared after switching “on” the junction Current (nA) TopographyConductance current (  A) Off On Off On

Transport Through Molecular devices 100 nm Single Molecule Measurements Nanoscale junctions 2  m Pt Ti Chang et al, APL (2003) T(K) C 18 H 36 OH Pt V Stewart et al, in preparation. diode r~5 x 10 5

Molecular Electronics Ultimate limit of miniaturization Self-assembly  low fabrication cost “Designer molecules”