Nano-electronics Exploring and exploiting novel electronic - PowerPoint PPT Presentation

CBSSS ‘04 Nano-electronics Exploring and exploiting novel electronic properties of nanomaterials Professor Marc Bockrath Applied Physics, Caltech for computation

Microelectronics The basis for present-day Circuitry patterned on micron length information technology scale Behavior of microelectronic devices well-described by classical physics, e.g. Ohm’s law

Moore’s Law 10000 1000 Feature size (nm) 100 10 1 0.1 1980 2000 2020 2040 2060 Year For nanometer-scale devices, as quantum mechanics (and other considerations unique to small structures) becomes important, we expect a rich variety of new transport phenomena to be observable

Nanoelectronics: Accessing the nanometer length scale Length Scale (nm) 1000 Chemical/biological 100 Synthesis: “bottom up” 10 Lithographic 1 techniques “top down” 0.1

Chemically/biologically Synthesized Nanostructures Single-walled nanotubes Nanocrystals Multi-walled nanotubes DNA Cobalt ion + ligands

Carbon Nanotubes Electronic Chemical/Biological Mechanical

Graphite – sp 2 C Carbon Diamond – sp 3

Nanotube rolled from graphite sheet

Measure 1 µ m Sample Fabrication Cr/Au leads Deposit Locate nanotubes 1 µ m

Operation of Single-walled Nanotube Devices SWNT Nanotube device geometry SiO 2 Gate Band structure G vs. V g G Metallic E F 0 SWNT E 0 V g k G Semiconducting E F SWNT 0 E 0 V g k

Nanotubes: a One-Dimensional Electron Box Low temperature behavior T=1.4 K 2.0 G ( µ S) 0.0 1.0 0.0 2.0 V g (V)

Carbon nanotubes – a one-dimensional electron box Energy spacing ∆ E between discrete levels determined in principle by solving Schrodinger equation ∆ E States are filled with electrons up to the E F Fermi level E F in accordance with the Pauli exclusion principle – two electrons per orbital assuming spin degeneracy Energy cost ∆ E to add electron to empty orbital relative to Fermi level

Nanotube electron box Coulomb repulsion Sets another energy scale U = e 2 / C Total energy to add electron: U + ∆ E Gate voltage can tune the relative position of the energy U + ∆ E gap to the Fermi level

Adding electrons one-by-one: single electron transistor Conductance Gate Voltage

Nanotube Transport Spectroscopy V g (arb units) -8.5 V g (V) -9.0 0.10 0.05 0.00 G ( µ S) 20 -20 0 V (mV)

Nanotube Transport Spectroscopy 20 V (mV) 0 -20 V g (arb units) 0.10 20 G ( µ S) 0.05 V (mV) 0 0.00 -20 0 20 V (mV) -20 Can measure directly U & ∆ E V g (arb units)

Single electron transistors: potential applications Classical & quantum Information storage & processing (see e.g., Likharev, Nakamura [cooper-pair box], Devoret & co-workers, etc.) High bandwidth charge detection (e.g. Schoelkopf and co-workers) Amplifiers Nanoscale motion sensors (Schwab and co- workers)

Variable Conductance Nanotube Device Conductance V g (Arb. units) 0 V (Arb. units)

Operation of a metallic Single-walled Nanotube Device 2 T =4 K 0 SWNT -2 V g (V) -4 SiO 2 Gate -6 -8 3.3 3.2 3.1 2.9 2.8 3.0 G ( e 2 / h )

High Conductance Nanotube Transport Measurements 2.9 dI/dV ( e 2 / h ) 5 V (mV) 0 -5 1.6 2 -2 0 V g (V) 3.3 5 dI/dV ( e 2 / h ) V (mV) 0 2.9 -5 2 -2 0 V g (V)

High Conductance Nanotube Transport Measurements 2.9 dI/dV ( e 2 / h ) 5 V (mV) 0 -5 1.6 L =500 nm 2 -2 0 V g (V) 3.3 5 dI/dV ( e 2 / h ) V (mV) 8 Vc (meV) 0 0 L =250 nm L -1 ( µ m -1 ) 2.9 0 5 -5 2 -2 0 V g (V)

Optical Resonator – Fabry-Perot Cavity Overall transmission of light determined by the time interference of partially reflected light waves Incident light position 1 Transmission oscillates Transmission as a function of the round trip phase 0.5 accumulation φ 0 0 2 4 6 φ / π

Electron resonator – Nanotube Cavity time Incident electron position • Potential energy of the electrons tunable by varying the gate voltage • Kinetic energy tunable by varying the bias voltage •This allows the tuning of the deBroglie wavelength of the electrons in the nanotube, resulting in the observed interference pattern

Comparison Between Data and Theory 8 V (mV) 0 -8 -2 -1 0 1 2 V g (V) 8 Reproduces all the major Vc (meV) features of the data Gives energy period of oscillations 0 with no free parameters 0 5 L -1 ( µ m -1 ) Nanotubes are ballistic, coherent electron waveguides! Bockrath et al. Nature 2001

Nanomechanical machines and computers 10 µ m Micromachine (from Sandia labs) Nanotube bearing motor (Zettl group, Nature ‘03) Nnaotube nanomechanical memory (Lieber group, Science ‘00) Mechanical computing paradigm e.g., Babbage ‘analytical engine’

Self-aligned nanotube linear bearings MWNT MWNT HF etch 250 I ( µ A) From H.-Y. Chiu 0 0 100 200 Time (s) Related work, P. Collins, J. Cumings et al.

Device current-voltage characteristics 15 I ( µ A) 0 -15 -4 0 4 V (V) From V. Deshpande Devices show hysteresis!

Forces acting on inner tube: Retraction Electrostatic Adhesion V 0 Bearing Electron extended Microscope images 100 nm

Potential applications of nanomechanical devices High-frequency : Logic gates Memories Oscillators Mixers Discriminators See e.g. Roukes etc.

Molecule-based Electronics • Ultimate limit of miniaturization • Self-assembly → low fabrication cost

Attaining the ultimate limit of miniturization Chemical synthesis of individual molecules allows construction of nanoscale objects with atomic precision Liang et al. Nature (’02) J. Park et al. Nature (’02)

Individual divanadium molecule transistors studied using electromigration- induced break-junction technique Park et al., APL (’98)

Kondo Effect in a single Divanadium Molecule Schematic Diagram (S=1/2) • Coherent superposition of virtual spin flip events leads to a narrow Kondo resonance near the Fermi energy of the leads. • The appearance of a Kondo resonance indicates spin degeneracy. Kondo effect in GaAs quantum dots: Goldhaber-Gordon et al . Nature (’98), Cronenwett et al . Science (’98)

Tunable Kondo Effect U ε E f Γ We can tune ε just by varying the gate voltage Additional energy scale k B T K

Tunable Kondo effect Kondo temperature exponentially decaying in ε, in accordance with theoretical predictions (e.g. Haldane et al. )

Another Molecular Wire? DNA similar in diameter to nanotubes Its recognition capabilities may enable self-assembly of nanoelectronic circuits But........

Does it Conduct?! YES Fink & Schonenberger Nature (1999) Kasumov et al. Science (2001) ETC. NO Porath et al. , Nature (2000) de Pablo et al ., PRL (2000) ETC.

Scanned Conductance Microscopy f 0 V Tip applies potential so as to induce local charge density Presence of absence of charge determined by monitoring the cantilever resonant frequency

Minimum Detectible Wire Conductance Charge motion takes a characteristic time AFM tip equal to the RC time constant of the wire This time constant is ~10-10 s for a 1M Ω 10 µ m long wire Tip scans over wire in characteristic time ~10-3 s Current 10 L ( µ m) 5 Can detect extremely low conductivity wires! 0 10 -10 10 -16 10 -22 G 0 ( Ω -1 – cm)

Result for λ -DNA Scanned Con ductance Topographic Image Image Nanotube λ -DNA 0.1 µ m No signal from the λ -DNA in the scanned Conductance image Bockrath et al . Nanoletters (’02)

Challenge: addressing individual nanodevices Nanowire Crossbar array – potential for high integration density V Problem: Voltage applied to one wire acts on all the crossing wires in parallel

Making junctions behave differently Crossed Si nanowires, one used as a gate electrode, the other as a MOSFET channel With Lieber group Zhong et al ., Science Untreated junction Treated with tetraethyl (2003) ammonium chloride

Can use this to make a decoder! Junctions can be selectively treated to enable independently controllable function

2x2 and 4x4 array Demonstration of Decoder with a

Project idea: Design a system of logic gates based on single-electron transistors Example: NOT gate Design XOR, NAND gates +V Also how about single electron logic? What are some of the major challenges that must be overcome if SET logic is to -V be achieved? What are the issues associated with achieving room From K. Likharev temperature operation?

Collaborators Nanotube Transport Scanned Conductance Microscopy Wenjie Liang Nina Markovic Hongkun Park Adam Shepard Michael Tinkham Leonid Gurevich Jason Hafner Leo Kouwenhoven Charles Lieber Minshaw Wu Lydia Sohn Nanotube Relay Vikram Deshpande Si nanowire wire decoder Hsin-Ying Chiu Zhaohui Zhong Deli Wang Molecule SET Yi Cui Charles M. Lieber Wenjie Liang Matthew Shores Jeffrey Long Hongkun Park