14/12/2006 HYDROMEL Training

State of the Art on Self-Assembly

Dominik Bell, Li Zhang, Lixin Dong, Bradley J. NelsonInstitute of Robotics and Intelligent Systems

Swiss Federal Institute of Technology (ETH), Zürich

Serge Palacin, Renaud DemadrilleDRECAM/Chimie des Surfaces et Interfaces

DRFMC/Electronique Moléculaire

Alan O’RiordanNanotechnology Group,

Tyndall National Institute, Prospect Row, Cork

Helmut Knapp, Raphael PuginCSEM

14/12/2006 HYDROMEL Training

Overview

• Self-assembly is the autonomous organization of components into patterns or structures without human intervention.

• Self-assembling processes are common throughout nature and technology.

• They involve components from the molecular (crystals) to the planetary (weather systems) scale and many different kinds of interactions.

• The concept of self-assembly is used increasingly in many disciplines, with a different flavor and emphasis in each.

• There are two main kinds of self-assembly: static and dynamic.

G. M. Whitesides and B. Grzybowski, Science 295, 2418, 2002

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The Definition of Self-Assembly

• Self-assembly is the autonomous organization of components into patterns or structures without human intervention.

—G.M. Whitesides, B. Grzybowski• Self-assembly is a spontaneous and reversible process that brings

together in a defined geometry randomly moving distinct bodies through selective bonding forces.

—Serge Palacin, Renaud Demadrill• “Self-assembly” is not a formalized subject, and definitions of the term “self-assembly”

seem to be limitlessly elastic. As a result, the term has been overused to the point of cliche. Processes ranging from the non-covalent association of organic molecules in solution to the growth of semiconductor quantum dots on solid substrates have been called self-assembly.

• Here, we limit the term to processes that • involve pre-existing components (separate or distinct parts of a disordered structure), • are reversible, and • can be controlled by proper design of the components.

• “Self-assembly” is thus not synonymous with “formation.”

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Basic Components

• The term components refers to the group of basic subunits of any self-assembly process.

• Together the components will form a superstructure overspecific interactions. The assembly process and the final structure can be said to be „encoded“ in the properties of theindividual subunits.

• The components need complementary properties such as a specific surface properties, charge, polarizability, magneticdipole, mass, surface functionalities.

• The number of components often largely depends on theirscale: in molecular SA processes 1020 can easily take partwhereas in macroscopic SA the number is much lower around10-104 components.

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A - TG - C

SA in Nature: DNA

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SA in Nature: Proteins

RNAstrand

Coatprotein

100 nm100 nm

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Scale of Self-assembly

• Self-assembly takes place at all scales.

• The following scales are typically distinguished:• Molecular and nanoscale self-assembly (classical form of SA

in chemistry involving atoms, molecules, and crystalformation)

• Intramolecular SA, e.g. protein folding (secondary and tertiary structure)

• Intermolecular SA, e.g. supramolecular micelle formation(quaternary structure), self-assembled monolayers(SAM)

• Meso- and macroscopic self-assembly (e.g. engineeredmicroparts)

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The Scale of Self-Assembly

Å

10-9

10-6

10-3

1

103

106

109

1012

1015

nm

μm

mm

m

km

Edge of the solar system

Earth to Mars

Nearest Stars

A 3D aggregate of micrometer plates assembled by capillary forces.

Convection cells formed above a micropatterned metallic support. The distance between the centers of the cells ~2 mm.

Self-assembled peptide amphiphile nanofibers.

A school of fish

Pluto

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Environment + Interaction Properties

• The free movement of components is required. • SA typically takes place in fluid phase or at a gas/fluid interface.• The forces which drive SA are typically relatively weak for the scale of

the components.• On the molecular level

• Covalent, but mostly non-covalent molecular forces involved (van der Waals, electrostatic, hydrogen bonds, hydrophobic/-philicinteractions).

• On the micro- and mesoscale:• Magnetic, capillary, electrostatic, and gravitational forces may be

exploited.• Reversibility: the interactions and connections between components

need to be sufficiently weak to be broken and reconfigured again in case of misconfiguration.

14/12/2006 HYDROMEL Training

Static Self-Assembly

• Static self-assembly involves systems that are at global or local equilibrium and do not dissipate energy.

• In static self-assembly, formation of the ordered structure may require energy (for example in the form of stirring), but once it is formed, it is stable.

• Examples of static self-assembly:• (A) Crystal structure of a ribosome. • (B) Self-assembled peptide-amphiphile

nanofibers. • (C) An array of millimeter-sized polymeric

plates assembled at a water/perfluorodecalininterface by capillary interactions.

• (D) Thin film of a nematic liquid crystal on an isotropic substrate.

• (E) Micrometer-sized metallic polyhedra folded from planar substrates.

• (F) A three-dimensional aggregate of micrometer plates assembled by capillary forces. PECVD for directed growth of vertically aligned nanotubes G. M. Whitesides and B. Grzybowski,

Science 295, 2418, 2002

14/12/2006 HYDROMEL Training

Dynamic Self-Assembly

• In dynamic self-assembly, the interactions responsible for the formation of structures or patterns between components only occur if the system is dissipating energy.

• The patterns formed by competition between reaction and diffusion in oscillating chemical reactions are simple examples; biological cells are much more complex ones.

• Examples of dynamic self-assembly: • (A) An optical micrograph of a cell with

fluorescently labeled cytoskeleton and nucleus; microtubules (~24 nm in diameter) are colored red.

• (B) Reaction-diffusion waves in a Belousov-Zabatinski reaction in a 3.5-inch Petri dish.

• (C) A simple aggregate of three millimeter-sized, rotating, magnetized disks interacting with one another via vortex-vortex interactions.

• (D) A school of fish. • (E) Concentric rings formed by charged metallic

beads 1 mm in diameter rolling in circular paths on a dielectric support.

• (F) Convection cells formed above a micropatterned metallic support. The distance between the centers of the cells is ~2 mm.

G. M. Whitesides and B. Grzybowski, Science 295, 2418, 2002

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Types of Self-Assembly

• S, static, D, dynamic, T, templated, B, biological

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Types of Directed Self-Assembly

• Chemical templates• Physical templates (e.g. holes, trenches)• Electrostatic fields• Electromagnetic fields• Fluidic forces• Capillary forces

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Self-Assembled Monolayers (SAMs)

• First developed in the early 1900s: Langmuir-Blodgett films

• Connection between substrate and reactive head group of units

• Physisorption, chemisorptionor covalent bonds

• Non-reactive tail group which stops the process after one layer is created

Preparation of SAMs. The substrate, Au on Si, is immersed into an ethanol solution of the desired thiol(s). Initial adsorption is fast (seconds); then an organization phase follows which should be allowed to continue for >15 h for best results. A schematic of a fully assembled SAM is shown to the right.

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SAMs in Fabrication: Principle

Patterned SAMs for the fabrication of structures in the range of 100 nm and less by etching and deposition.

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SAMs in Fabrication: Surfacespatterned with the help of SAMs

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Self-Assembled Surface Micelles

• Switchability of surfaces • Core-corona inversion of self-assembled block-copolymer surface micelles• Solvent induces reorganisation and thus change in surface properties

contact angle89 ° (adv)65 ° (rec)

contact angle65 ° (adv)<20 ° (rec)

methanol

toluene

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DNA as Building Block: DNA Origami

Self-assembling smileys (~100nm) based on folding DNA (Paul Rothemund of the California Inst. of Technology)

DNA crosslink between helices by sharing 2 strands, planning of patternformation, and AFM image of a real DNA pattern (Brun et al. 2006)

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Directed Self-Assembly with ChemicalTemplates for Microfabrication

Hydrophilic/hydrophobic

Clark, T. D.; Tien, J.; Duffy, D. C.; Paul, K. E.; Whitesides, G. M. J. Am. Chem. Soc. 2001, 123 (31), 7677 -7682.

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Types of Directed Self-Assembly

• Chemical templates• Physical templates (e.g. holes, trenches)• Electrostatic fields• Electromagnetic fields• Fluidic forces• Capillary forces

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Directed Self-Assembly with PhysicalTemplates

Silicon technology (microelectronics) is not compatible with GaAs technology

Integration of GaAs on Si achieved by fluidic self-assembly

Yeh HJJ, Smith JSIEEE Photonics Technol. Lett. 6 (1994) 706-708

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W. Zheng, H.O.Jacobs Adv.Funct.Mater. 2005, 15, 5

600 AlGaInP/GaS LED segments Assembled in 2 minutes withoutdefects

Directed Self-Assembly with PhysicalTemplates

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Types of Directed Self-Assembly

• Surface forces / chemical templates• Physical templates (e.g. holes, trenches)• Electrostatic fields• Electromagnetic fields• Fluidic forces• Capillary forces

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Field Assisted Trapping and Self-assembly of Nanowires

Parallel and orthogonal assembly of nanowires with electric fields, electrodes biased at 50±100 V after a drop of nanowire solution is deposited on the substrate. C.M. Lieber, Nature 409 (2001) 66.

Optical micrograph image of 5 μm long, 200 nm diameter gold nanowires aligned by applying 30 V at 1 kHz to the underlying comb electrode structures T.E. Mallouk, Appl. Phys. Lett. 77 (2000) 1399.

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Dielectrophoretic Assembly of CNTs

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Define nanoelectrodes (Cr / Au) by EBL and lift – off

~Apply electric field Apply MWNT suspension

500nm oxide

Si chip

Blow droplet with N2 gunRemove electric field

Cr / Au by EBL and lift – off

curre

nt

voltage

breakdown

drive current

70

75

80

85

0 10 20 30 40 50

Time [s]

Cur

rent

[ μA

]

A Batch Fabrication Process Using DEP and Shell Engineering

ETHZ

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Types of Directed Self-Assembly

• Surface forces / chemical templates• Physical templates (e.g. holes, trenches)• Electrostatic fields• Electromagnetic fields• Fluidic forces• Capillary forces

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(a) Plan-view SEM image of a Pt-Ni-Pt nanowire trapped by magnetic fields from elliptical Ni electrodes. The vertical bars are Au leads for electrical measurements. Scale bar, 10 μm. (b) Close-up SEM image of the trapped nanowire; substrate tilted at 70°. Scale bar, 1 μm, D.H.Reich, J. Appl. Phys. 91 (2002) 8549.

Self-Assembly of Nanowires with Electromagnetic Fields

Assembly of 12 μm long nickel nanowires, functionalised with porphyrins and aligned in a magnetic field in a 1:1hexadecane/octadecane mixture.G.J. Meyer, Nano Lett 1 (2001) 155.

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Directed Self-Assembly with Electromagnetic fields

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Types of Directed Self-Assembly

• Surface forces / chemical templates• Physical templates (e.g. holes, trenches)• Electrostatic fields• Electromagnetic fields• Fluidic forces• Capillary forces

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Fluidic Self-assembly of Nanowiresin Microchannels

(a-c) Layer-by-layer assembly of InP nanowires aligned by fluid flow in capillaries. (d) Assembled and electrically contacted 2 x 2 array. C.M. Lieber, Science 291 (2001) 630.

Droplet move through channel by thermocapillarymotion. Au NWs are aligned by fluid flow and are adsorbed to gas surface. E. Zussman, Physics of Fluids 17 (2005) 063301-1.

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Types of Directed Self-Assembly

• Surface forces / chemical templates• Physical templates (e.g. holes, trenches)• Electrostatic fields• Electromagnetic fields• Fluidic forces• Capillary forces

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Capillary Forces

• Surface tension in dots of solder can also be used to induce such stress and to make 2D structures fold into 3D assemblies – vaguely similar to protein folding, a typical form of intramolecular assembly.

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Capillary Forces

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Capillary Forces

• Driving Force: Capillary interactions between solder dots on subunits in a liquid environment make them self-assemble

3D electronic circuit(Whitesides et al.)

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Capillary Forces

PDMS parts at water/PFD interface, coming together and interacting due to meniscus forces. The allosteric part is the horse-shoe shaped red component that is ‘opened’, or activated when the catalyst (4-lobed blue part) binds it’s functional site. The bound blue circular part can only fit into the horseshoe once the catalyst is bound (Saul Griffith 2002).

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Summary of Directed Self-Assembly

1 nm 1 μm 1 mm

Chemical templatesPhysical templatesElectrostatic fieldsElectromagnetic fieldsFluidic forces

Capillary forces

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Hybrid Assembly

Fx = 8.87x10-7 NFy = -1.10x10-10 N

Fx = 7.79x10-10 NFy = -9.98x10-13 N

Stable equilibrium achieved

A. Subramanian, B. Vikramaditya, L.X. Dong, D. Bell, and B.J. Nelson, Robotics: Science and Systems I, pp.327-334, MIT Press (2005)

• Limited achievable complexity of superstructures through directed self-assembly

•To increase complexity or productivity

→ hybrid approach

Hydromel

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Complexity and ProductivityPr

oduc

tivity

Complexity

Robotic Assembly

Self-assemblyDirected

Self-assembly

Parallel

Robotic Assembly

Hybrid

Assembly

Self-

Replication

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Summary

• Self-assembly is the autonomous organization of components into patterns or structures without human intervention.

• Self-assembly takes place at all scales and the boundaries are often blurred.

• Self-assembly as a part of self-organisation is omnipresent in nature

• There are static and dynamic self-assembly• There are different types of directed self-assembly that may be

limited to certain size-scales due to their driving forces• Hybrid approach combining self-assembly and robotic assembly

can result in higher complexity or productivity