VIII. Molecular Electronics and Nanoscience

Molecular Electronics and Nanoscience Why Molecular Electronics

Moore’s Law Devices: Top-down and Bottom-Up

Fabrication Single Molecule Systems and Materials Many-Molecule Systems and Thin Films

DNA Computing

•Role of contacts

•Molecular device

Contact Contact

Molecule

what type of behaviour we can expect for a

•complete system?

•Ballistic conductor •Contacts of finite transparency

•P1

•P2

The model:

•– Electrons tunnel with some probability (contact •transparency) into the channel

•– Transport is coherent

•– Contacts are ”reflectionless”

The question:

What is the resistance of the channel? Where the heat is •dissipated?

•Ballistic conductor model

Electrons moving from left to right have

•potential µ1, from right to left µ2.

Why Molecular Electronics

Moore’s Law

Silicon and Moore’s Law • Heat dissipation.

– At present, a state-of-the-art 500 MHz microprocessor with 10 million transistors emits almost 100 watts--more heat than a stove-top cooking surface.

• Leakage from one device to another. – The band structure in silicon provides a wide range of allowable

electron energies. Some electrons can gain sufficient energy to hop from one device to another, especially when they are closely packed.

• Capacitive coupling between components. • Fabrication methods (Photolithography).

– Device size is limited by diffraction to about one half the wavelength of the light used in the lithographic process.

• ‘Silicon Wall.’ – At 50 nm and smaller it is not possible to dope silicon uniformly. (This

is the end of the line for bulk behavior.)

Silicon and Moore’s Law • Moore’s second law.

– Continued exponential decrease in silicon device size is achieved by exponential increase in financial investment. $200 billion for a fabrication facility by 2015.

• Transistor densities achievable under the present and foreseeable silicon format are not sufficient to allow microprocessors to do the things imagined for them.

Moore’s “Second Law"

Plant cost Mask cost

generation

X 1

00

0$

Why Molecular Electronics?

• If current trend continues, it will reach molecular scale in

two decades.

• There are many molecules with interesting electronic properties.

Semiconductor devices shrink to the nano-scale

Year

1950 1960 1970 1980 1990 2000 2010

1 cm

1 m

100 nm

1-5nm

Tra

nsi

stor

Siz

e

Devices: Top-down and Bottom-up Fabrication

Electronics Development Strategies

• Top-Down.

– Continued reduction in size of bulk semiconductor devices.

• Bottom-up (Molecular Scale Electronics).

– Design of molecules with specific electronic function.

– Design of molecules for self assembly into supramolecular structures with specific electronic function.

– Connecting molecules to the macroscopic world.

Bottom-Up (Why Molecules?)

• Molecules are small. – With transistor size at 180 nm on a side, molecules are some 30,000 times

smaller. • Electrons are confined in molecules.

– Whereas electrons moving in silicon have many possible energies that will facilitate jumping from device to device, electron energies in molecules and atoms are quantized - there is a discrete number of allowable energies.

• Molecules have extended pi systems. – Provides thermodynamically favorable electron conduit - molecules act as

wires. • Molecules are flexible.

– pi conjugation and therefore conduction can be switched on and off by changing molecular conformation providing potential control over electron flow.

• Molecules are identical. – Can be fabricated defect-free in enormous numbers.

• Some molecules can self-assemble. – Can create large arrays of identical devices.

• Top-down Synthesis

1. Lithography

Developing Positive Negative

Etching and Stripping

Polymer Resist

Thin Film Substrate

Resist Resist

Exposing Radiation

Figure 1.1. Schematic of positive and negative resists.

Figure 1.6. Schematic of a focused ion beam system.

1 m

400 nm

300 nm

200 nm

160 nm

120 nm

100 nm

80 nm

60 nm100 nm

Carbon Nanotubes/Nanocones with Various Catalyst Patterning Dimensions by E-beam Lithography

Molecular Self-Assembly

• Self-Assembly on Metals

– (e.g., organo-sulfur compounds on gold)

• Assembly Langmuir-Blodgett Films

– Requires amphiphilic groups for assembly

• Carbon Nanotubes

– Controlling structure

Figure 2.1. The process of forming a self-assembled monolayer. A

substrate is immersed into a dilute solution of a surface-active

material that adsorbs onto the surface and organizes via a self-

assembly process. The result is a highly ordered and well-packed

molecular monolayer. (Adapted from Ref. 9 by permission of

American Chemical Society.)

Cyclic Peptide Nanotubes as Scaffolds for Conducting Devices

Hydrogen-bonding interactions promote stacking of cyclic peptides

Pi-systems stack face-to-face to allow conduction along the length of the tube

Cooper and McGimpsey - to be submitted

CYCLIC BIOSYSTEMS

Spontaneous self-directed chemical growth allowing

parallel fabrication of identical complex functional structures.

Self-assembly

.Characterization and Handling of

Ultra-small particles or Assemblies

–a. Optical Tweezers

–b. Electromagnetic tweezers

–c. In nanotemplates

–d. Structural Analysis by TEM,

SEM, X-ray, etc.

Ballistic Nanotube MOS Transistors (Chen,Hastings)

Wd

D

L

SWNTSWNT

SiO2

Source

Al-Gate

Ti

HfO2

Drain

L

L~20 L~20 nmnm

Placement of Nanotubes by E-Field

(The first-demo) Nanotube Field-Effect Transistor(FET)

E-Beam Lithography

• Measurements

Figure 3.1. Schematic showing all major components of an STM. In

this example, feedback is used to move the sensor vertically to

maintain a constant signal. Vertical displacement of the sensor is

taken as topographical data

Coarse approach

mechanism

S

c

a

n

n

e

r

Sensor

Sample

Reference

-

Signa l

feedback

data

Figure 3.1. Schematic showing all major

components of an SPM. In this example,

feedback is used to move the sensor

vertically to maintain a constant signa l.

Vertical displacement of the sensor is takenas topograph ical data.

•1980’s

Single Molecule Detection.

How to image at the molecular level.

How to manipulate at the molecular

level.

• Scanning Probe Microsopy. STM (IBM Switzerland, 1984)

AFM

Molecules as Electronic Devices:

Historical Perspective

Major equipment

• Focused Ion Beam System (FIB) (scheduled for installation in mid 2007) • Atomic Layer Deposition System (ALD) • Rapid Thermal Processing System (RTP) • Plasma Enhanced Chemical Vapor Deposition System (PECVD) • Standard Resolution Electron Beam Lithography (EBL) • Atomic Force Microscope for Nanopatterning, and Manipulation (AFM) • Atomic Force Microscope for Atomic Resolution Imaging (AFM) • Quartz Crystal Microbalance (QCM) • 4-furnace bank of 3-zone oxidation, dopant diffusion, and annealing furnaces • Class 100 Clean Room • Spin-Coating Station • Photolithography System • Surface Profiler • Chemical Treatment Station (cleaning, etching, and functionalization) • Ion Milling System • Plasma Cleaning/Oxidation System • Gas Cabinet Bank • Experimental Materials Thermal Evaporator • Standard Materials Thermal Evaporator • Electron-Beam Evaporator • Multi-target Sputtering System • Probe Station and Device Characterization System • Four-Point Resistance Measurement System • Ellipsometer • Optical Microscopes • Dicing Saw • Equipment Cooling Systems (3) • Inductive Coupled Plasma (ICP) Etching System (scheduled for installation in Feb. 2006) • Experimental materials sputtering system (scheduled for installation in mid 2006) • Ultra-High Resolution EBL and SEM System

Clean Room

Photolithography

Rapid Thermal

Processing

Quartz MicroBalance

Plasma

Enhanced

Chemical

Vapor

Deposition

Reactive Ion Etching

Atomic

Layer

Deposition

IV. Nanotemplates

• G. Inorganic

• H. Organic

Fig.2 (a) Nanostructure of anodically formed Al2O3 template. (b) its cross-section, (c) catalyst deposited at the bottom of the pores, (e) vertically aligned nanotubes, and (f) TEM

image of a nanotube.

(Chen, Singh, DeLong, Saito, Yang, Bhattacharyya, and Sumanasekeras)

Nano-scale Material Research

(a)

(b)

(c)

Catalyst

200nm200 nm

Vertically aligned MWNTs

embedded in AAO insulator

(d)

Si substrate

SiO2

SiO2

Carbon nanotubes

AlSiO2

Hexagonal Cells

Nano-template

Horizontally

aligned

The first vertically aligned nanotubes on silicon substrates using templates

• Fig. 3 Schematic representing the helix-coil transitions within the pore of a Poly-L-Glutamic Acid functionalized membrane (a) random-coil formation at PH > 5.5 , (b) helix formation at low pH ( <4 ).

Single Molecule Devices

Using a Single Molecule

Cornell group

What Might Single Molecule Devices Do?

Single-Electron Memory Cell

Fe+3

Fe+2

Heme group

e-

e- Au

Molecular Abacus

The “bead” can be reversibly

switched between two positions

by pH. Ashton et al. JACS, 120, 11932(1998)

Some Fancy Molecules

Rotaxane

Catenane Pretzelane Handcuffcatenane

Synthesis of a Rotaxane Molecule

Amabilino and Stoddart, Chemical Reviews 95, 2725 (1995).

Angew. Chem., Int. Ed. 2000, 39, 3284-3287.

Molecular Motor

Molecular Oscillator

Molecular Sensor

K+K+

Molecular Recognition: A capability that Si lacks

K +

Ag + K+K+ K

+ Ag

+

Crown ether

• A small difference in the diameters of the K+ and Ag+

can cause a huge difference in the binding capacity

Heath and Ratner, Physics Today, May 2003, p. 43

Many Ideas for Single Molecule Devices

Nano-switch

Single Molecule Systems and Materials

Molecular conduction

molecule

Molecules as Electronic Devices: Historical Perspective

•1970’s: Single Molecule Devices?

• In the 1970’s organic synthetic

techniques start to grow up

prompting the idea that device

function can be combined into a

single molecule.

• Aviram and Ratner suggest a

molecular scale rectifier. (Chem.

Phys. Lett. 1974)

• But, no consideration as to how

this molecule would be

incorporated into a circuit or

device.

Molecular Rectifiers

Arieh Aviram and Mark A. Ratner IBM Thomas J. Watson Research Center, Yorktown Heights, New

York 10598, USA

Department of Chemistry, New York New York University, New

York 10003, USA

Received 10 June 1974 Abstract

The construction of a very simple electronic device, a

rectifier, based on the use of a single organic molecule is

discussed. The molecular rectifier consists of a donor pi

system and an acceptor pi system, separated by a sigma-

bonded (methylene) tunnelling bridge. The response of such

a molecule to an applied field is calculated, and rectifier

properties indeed appear.

Acceptor Donor

Single Molecule Systems and Materials

Elastic

Inelastic V

h/e V

h/e V

h/e V

I d

I/d

V

d2I/d

V2

h/e

Finding a true molecular signature: Inelastic Electron Tunnelling Spectroscopy (IETS)

Towards Single Molecule Electronics

Can a single molecule behave like a

diode, transistor (switch), memory ?

If that’s possible, how long will the molecule last ?

First, let’s look at many molecules acting in parallel.

Nitzan and Ratner, Science 300, 1384 (2003); Heath and Ratner, Physics Today, May 2003, p. 43

Single-Molecule Conductivity L ELECTRODE R ELECTRODE MOLECULE

L ELECTRODE R ELECTRODE MOLECULE

Fermi energy

Molecular Orbitals

eV

V

L ELECTRODE R ELECTRODE MOLECULE

I

Molecular Orbitals

Molecular Electronics: Measuring single molecule conduction

Kushmerick et al. PRL 89

(2002) 086802

Cross-wire

Wang et al.

PRB 68 (2003) 035416

Nanopore STM Break Junction

B. Xu & N. J. Tao Science (2003) 301, 1221

Electromigration

H. S. J. van der Zant et al.

Faraday Discuss. (2006)

131, 347

Nanocluster

Dadosh et al. Nature

436 (2005) 677

Scanning Probe

Cui et al.

Science 294

(2001) 571

Reichert et al. PRL 88

176804

Mechanical Break

Junction

Mechanically-Controlled Break Junction

Resistance is a few megohms.

(Schottky Barrier)

Molecular Junction

A schematic representation of Reed and Tour’s molecular junction containing a

benzene-1,4-dithiolate SAM that bridges two proximal gold electrodes.

Break Junctions

At the beginning of single molecule

electronics, break junctions were very

popular: Just crack a thin Au wire open

in a vice and adjust the width of the

crack with piezos (as in STM). Then

pour a solution of molecules over it.

Alternatively, one can burn out the

thinnest spot of a thin Au wire by

running a high current density through

it (using the effect of electromigration).

These days, many try to achieve a

well-defined geometry using a STM

or AFM, with a well-defined atom at

the end of the tip and another well-

defined atom at the surface as con-

tacts to a single molecule.

Nanotube conductivity is quantized.

Nanotubes found to conduct current ballistically and do not dissipate heat.

Nanotubes are typically 15 nanometers wide and 4 micrometers long.

Carbon Nanotubes

Gentle contact needed

Using a Few Molecules

Observe tunneling through 1, 2, 3, 4, 5 alkanethiol molecules

Cui et al., Science 294, 571 (2001)

“The resistance of a single octanedithiol molecule was 900 50 megaohms, based on measurements on more than 1000 single molecules. In contrast, nonbonded contacts to octanethiol monolayers were at least four orders of magnitude more resistive, less reproducible, and had a different voltage dependence, demonstrating that the measurement of intrinsic molecular properties requires chemically bonded contacts”.

Cui et al (Lindsay), Science 294, 571 (2001)

Dynamics of current voltage

switching response of single

bipyridyl-dinitro oligophenylene

ethynylene dithiol (BPDN-DT)

molecules between gold contacts.

In A and B the voltage is changed

relatively slowly and bistability

give rise to telegraphic switching

noise. When voltage changes

more rapidly (C) bistability is

manifested by hysteretic behavior

Lortscher et al (Riel), Small, 2, 973 (2006)

Chem. Commun., 2006, 3597 - 3599, DOI: 10.1039/b609119a

Uni- and bi-directional light-induced switching of diarylethenes on gold

nanoparticles

Tibor Kudernac, Sense Jan van der Molen, Bart J. van Wees and Ben L.

Feringa

“In conclusion, photochromic

behavior of diarylethenes

directly linked to gold nanoparticles

via an aromatic spacer has

been investigated. Depending on the

spacer, uni- (3) or bidirectionality

(1,2) has been observed.”

Switching with light

Current–voltage data (open circles) for (a) open

molecules 1o and (b) closed molecules 1c

Nanotechnology 16 (2005) 695–702

Switching of a photochromic molecule on gold electrodes: single-molecule

measurements

J. He, F. Chen, P. Liddell, J. Andr´easson, S D Straight, D. Gust, T. A. Moore,

A. L. Moore, J. Li, O. F Sankey and S. M. Lindsay

Conductance through a C60 Molecule

Distance dependence tells whether it is tunneling (exponential decay)

or quantum conductance through a single or multiple orbitals (G0).

Kröger et al., J. Phys. Condend. Matter 20, 223001 (2008)

(a) Structures of the long and short linked cobalt coordinated terpyridine thiols used

as gate molecules. (b) A topographic AFM image of the gold electrodes with a gap.

(c) A schematic representation of the assembled single atom transistor.

A Molecular Transistor

Many-molecule Systems and Thin Films

mNDR = molecular Negative Differential Resistance

Measured using a conducting AFM tip

Negative Differential Resistance

One electron reduction provides a charge carrier.

A second reduction blocks conduction.

Therefore, conduction occurs only between the

two reduction potentials.

Voltage-Driven

Conductivity Switch

Applied perpendicular field favors

zwitterionic structure which is planar

Better pi overlap, better conductivity.

Dynamic Random Access

Memory

Voltage pulse yields

high conductivity

State - data bit stored

Bit is read as high

in low voltage region

Device is fabricated by sandwiching a layer

of catenane between an polycrystalline layer of n-doped

silicon electrode and a metal electrode. The switch is

opened at +2 V, closed at -2 V and read at 0.1 V.

Voltage-Driven

Conductivity Switch

High/Low Conductivity Switching Devices Respond to I/V Changes

Voltage-Driven

Conductivity Switch

n-type

Voltage-Driven

Conductivity Switch

Other Molecular Switches

Chen et al., Science 286, 1550 (1999)

Large On-Off Ratios

Data Storage via the Oxidation State of a Molecule

Electrochemistry

40 nm line width, 40 Gbit/inch2

HP Molecular Memory

Output: Stored Data

Input: Address

Molecular Memory

MRAM (Magnetic Random Access Memory)

Crossbar Memory

Architecture

DRAM

1

0

HP Molecular Memory

The blue ring can shuttle back

and forth along the axis of the

rotaxane molecule, between

the green and red groups.

Rotaxane molecules switch

between high and low resis-

tance by receiving a voltage

pulse.

Collier et al., Science 289, 1172 (2000).

(Many Molecules)

HP Molecular Memory

Change the resistance between

low and high by voltage pulses.

Is the resistance change really due

to the rotaxane ring shuttling back

and forth? Other molecules exhibit

the same kind of switching.

One possible model is the creation

and dissolution of metal filaments

which create a short between the

top and bottom electrodes. (Some-

thing like that happens in batteries).

Robert F. Service,

Science 302, 556

(2003).

Quantum

Dot

Molecular Switch

Self-Organizing Memory + Data Processor

Heath et al., Science

280, 1716 (1998)

People have been thinking about

how to combine memory with logic

(= a microprocessor) in a molecular

device.

Self-assembly is the preferred

method. It generates errors, though.

They need to be absorbed by a

fault-tolerant architecture (e.g. in the

HP Teramac)

DNA Computing

DNA Computing

I believe things like DNA computing will eventually lead the way to a “molecular revolution,” which ultimately will have a

very dramatic effect on the world. L. Adleman

What is DNA?

• All organisms on this planet are made of the same type of genetic blueprint.

• Within the cells of any organism is a substance called DNA which is a double-stranded helix of nucleotides.

• DNA carries the genetic information of a cell.

• This information is the code used within cells to form proteins and is the building block upon which life is formed.

• Strands of DNA are long polymers of millions of linked nucleotides.

Graphical Representation of inherent bonding properties of DNA

Double Helix shape of DNA

The two strands of a DNA molecule are anti parallel where each strand runs in an opposite direction.

GC base pair and AT base pair

Adleman’s Experiment

• Hamilton Path Problem (also known as the travelling salesperson problem)

Perth

Darwin

Brisbane

Sydney

Melbourne

Alice Spring

Is there any Hamiltonian path from Darwin to Alice Spring?

Adleman’s Experiment (Cont’d)

• Solution by inspection is: Darwin Brisbane Sydney Melbourne Perth

Alice Spring

• BUT, there is no deterministic solution to this

problem, i.e. we must check all possible

combinations.

Perth

Darwin

Brisbane

Sydney

Melbourne

Alice Spring

Adleman’s Experiment (Cont’d)

1. Encode each city with complementary base -

vertex molecules Sydney - TTAAGG

Perth - AAAGGG

Melbourne - GATACT

Brisbane - CGGTGC

Alice Spring – CGTCCA

Darwin - CCGATG

Adleman’s Experiment (Cont’d)

2. Encode all possible paths using the

complementary base – edge molecules Sydney Melbourne – AGGGAT

Melbourne Sydney – ACTTTA

Melbourne Perth – ACTGGG

etc…

Adleman’s Experiment (Cont’d)

3. Merge vertex molecules and edge molecules. All complementary base will adhere to each other to

form a long chains of DNA molecules

Solution with

vertex DNA

molecules

Solution with

edge DNA

molecules

Merge

&

Anneal

Long chains of DNA molecules (All

possible paths exist in the graph)

Adleman’s Experiment (Cont’d)

• The solution is a double helix molecule:

CCGATG – CGGTGC – TTAAGG – GATACT – AAAGGG – CGTCCA

TACGCC – ACGAAT – TCCCTA – TGATTT – CCCGCA

Darwin Brisbane Sydney Melbourne Perth Alice Spring

Darwin Brisbane

Brisbane Sydney

Sydney Melbourne

Melbourne Perth

Perth Alice Spring

Basics And Origin of DNA Computing

• DNA computing is utilizing the property of DNA for massively parallel computation.

• With an appropriate setup and enough DNA, one can potentially solve huge problems by parallel search.

• Utilizing DNA for this type of computation can be much faster than utilizing a conventional computer

• Leonard Adleman proposed that the makeup of DNA and its multitude of possible combining nucleotides could have application in computational research techniques.

Problems with Adleman’s Experiment

• The researchers performed Adleman’s Experiment and the results obtained were inconclusive.

• The researchers state that “At this time we have carried out every step of Adleman’s Experiment but have not gotten an unambiguous final result.”

• The problem is because of the underlying assumption that the biological operations are error-free.

Problem Instance

• There are 2 problems with extraction: – The removal of strands containing the sequence in not 100% efficient.

– May at times inadvertently remove strands that do not contain the specified sequence.

• Adleman’s did not encounter problems with extraction because only a few operations were required.

• However, for a large problem instance , the number of extractions required may run into hundreds or even thousands.

• Time- Adleman talked of a week of work in lab, but tuning

such an experiment could take one month work

• Contradictory results- We do not know a lot of experiments like Adleman’s, nor Adleman’s trials of repeating the experiment.

Problems Contd.

Advantages of a DNA Computer

• Parallel Computing- DNA computers are massively parallel.

• Incredibly light weight- With only 1 LB of DNA you have more computing power than all the computers ever made.

• Low power- The only power needed is to keep DNA from denaturing.

• Solves Complex Problems quickly- A DNA computer can solve hardest of problems in a matter of weeks

Disadvantages of DNA Computer • High cost is time.

• Occasionally slower-Simple problems are solved much faster on electronic computers.

• It can take longer to sort out the answer to a problem than it took to solve the problem.

• Reliability- There is sometime errors in the pairing of DNA

strands

DNA Chip

Source: Stanford Medicine Magazine, Vol 19, 3 Nov 2002 http://mednews.stanford.edu/stanmed/2002fall/translational-dna.html

Chemical IC

Source: Tokyo Techno Forum 21, 21 June 2001 http://www.techno-forum21.jp/study/st010627.htm

The Smallest Computer

• The smallest programmable DNA computer was developed at Weizmann Institute in Israel by Prof. Ehud Shapiro last year

• It uses enzymes as a program that processes on on the input data (DNA molecules).

• http://www.weizmann.ac.il/mathusers/lbn/new_pages/Research_Biological.html

References

• “Molecular Computation of Solutions to Combinatorial

Problems”, L.M. Adleman, Science Vol.266 pp1021-1024,

11 Nov 1994

• “Computing With Cells and Atoms – an introduction to

quantum, DNA and membrane computing”, C.S. Calude and

G. Paun, Taylor & Francis, 2001

• “The Cutting Edge Biomedical Technologies in the 21st

Century”, Newton, 1999

• “Human Physiology: From Cells to Systems 4th Ed.”, L.

Sherwood, Brooks/Cole, 2001