Nanofabrication: Self Assembly “The spontaneous association of molecules under equilibrium conditions into stable, structurally well-defined aggregates” ME584 Introduction to Nanotechnology Nick Bublavi, Kevin Voss, Greg Behrmann April 29, 2005

Outline Introduction Important Properties at Small Scales Some Self Assembly Mechanisms Macro Scale Self Assembly Demonstrations Technical Challenges and Future Work Some Companies Involved in Self Assembly Based Nanotechnology

Introduction Self Assembly is analogous to the assembly of a puzzle where the characteristics of each piece and the surrounding environment allow each piece to only fit in its proper location. see Soft Machines: Nanotechnology and Life, Richard A. L. Jones, Oxford Press, 2004

Introduction Why is Self Assembly an important fabrication method for nanotechnology? Semiconductor Fabrication Methods are “Top-Down” and require a large number of steps, an expensive infrastructure, and feature sizes are limited by diffraction limits of exposure wavelength. see www.mcgill.ca / chemistry / undergrad / courses / 534.html

Introduction Why is Self Assembly an important fabrication method for nanotechnology? While offering high resolution and nanometer scale feature sizes, Direct Write Methods such as e-beam lithography, focused ion beam, and atom force manipulation operate over small areas in a serial manner making them slow and economically undesirable for production in large volumes. E-beam Write Areas are ~0.5 µm to 2 mm square with stitching errors between write fields ~20-40 nm see www.raith.com Nanolithography products

Introduction Why is Self Assembly an important fabrication method for nanotechnology? As opposed to the “Top Down” methods of the semiconductor industry, Self assembly exploits the naturally existing effects of Brownian Motion, Intermolecular Forces, and the 2nd Law of Thermodynamics to produce structures in a “Bottom Up” fashion. Puzzle Analogy One Pot Cooking

Properties at Small Scales Brownian Motion The random movement of microscopic particles by thermal energy and collisions with molecules of the surrounding medium. Gas Molecules in a Container See http://www.phy.ntnu.edu.tw/java/gas2D/gas2D.html

Properties at Small Scales Brownian Motion Observed in 1827 by Robert Brown in observation of pollen molecules. Mathematically described by Albert Einstein in 1905. A single water molecule at room temperature has an average velocity of 600 miles per hour. A 100 nm size particle such as a bacterium has an average velocity of 120 miles per hour. A 1 micron size particle has an average velocity of 4 miles per hour. Random Walk/Diffusion- implies that the motion is undirected and does not cover large distances. An oxygen molecule in water will move 10 nm in a few milliseconds, 1 minute to move a micron, and 100 years to move a centimeter. Brownian Motion is also used in financial modeling. see Soft Machines: Nanotechnology and Life, Richard A. L. Jones, Oxford Press, 2004

Properties at Small Scales Intermolecular Forces All matter is held together by forces. The force between atoms within a molecule is a chemical or intramolecular force. The force between molecules is a physical or intermolecular force. See http://itl.chem.ufl.edu/2045/lectures/lec_g.html

Properties at Small Scales Intermolecular Forces Intermolecular forces can be considered electromagnetic in nature and weaker than the forces within molecules. The physical properties of melting point, boiling point, viscosity, surface tension, and solubility are related to the strength of attractive forces between molecules. At the nanoscale, intermolecular forces cause particles to want to “stick” to each other. See http://www.science.uwaterloo.ca/~cchieh/cact/c123/intermol.html

Essence of Self Assembly Successful self assembly engineering requires the balance of Brownian Motion and Intermolecular Forces. Brownian Motion contributes by moving components into correct location and orientation. Intermolecular Forces are utilized to hold components in position. Process is massively parallel and “self-correcting”. If a component is assembled incorrectly, it is likely that Brownian Motion will shake it loose until it assumes the proper configuration. A Reversibly Switching Surface

Self Assembly Methods Hydrophilic/Hydrophobic Molecules Molecules containing polar charges are able to form hydrogen bonds to water. These molecules are hydrophilic. e.g. sugar, acetic acid, aspirin Molecules that are uncharged and contain no polar groups (non-polar) and are therefore unable to form hydrogen bonds to water are hydrophobic. e.g. engine oil, olive oil. Lipids are molecules that possess a hydrocarbon tail that is hydrophobic, or insoluble in water and a polar head group that is hydrophilic, or water soluble. The total length of the lipid molecules is 2-4 nm. As building blocks, supramolecular structures can be built that are hundreds of nanometres to hundreds of microns in size.These structures are known as amphiphiles. See http://distance.stcc.edu/AandP/AP/AP1pages/Units1to4/epitissmol/lipids.htm And http://www.chems.msu.edu/classes/s03/891/003/

Self Assembly Methods Hydrophilic/Hydrophobic Molecules At MIT, researchers have developed self assembly techniques that produce nanotubes and spheres from peptides by exploiting hydrophobic/hydrophilic properties with ionic bonding. See Zhang, S., Materials Today, May 2003.

Self Assembly Methods SAMS: Self Assembled Monolayers Self assembly techniques have been developed that use Soft Lithography as mechanism for depositing self-assembling layers on the nanoscale. Kumar, et al “Patterning Self Assembled Monolayers: Applications in Material Science,” Langmuir, Vol. 10, No. 5, 1994

Self Assembly Methods Self Assembly of Magnetic Particles Techniques have been developed that use iron nano particles that self assemble into 3D arrays . See http://www.nanonet.go.jp/english/mailmag/2004/030b.html

Self Assembly Methods Self Assembly by Centrifugation and Spin-annealing Spin coating has been used to produce three dimensional close packed lattices used as photonic crystals. See http://www.ece.udel.edu/~dprather/research/pbg/fabrication/3dpbgfab_selfassemble.pdf

Macro Scale Self Assembly Demos DISCLAIMER: The following demonstrations are Macro-Scale and their purpose is only to illustrate the how self assembly mechanisms work. At the nanoscale, due to size, the physics can be quite different. Two atomic crystal configurations gives NiTi shape memory Polarity causes magnets to assemble in alternating colors Hydrophobic grape skin floats. Hydrophilic peeled grape sinks in soda .

Future Work Control of in-plane dimensions for single layers less than 20 nm. Development of the theory of designable self assembling machine structures. Develop computational modeling of the self assembly process. Develop and incorporate self assembly capability at the interface of different nanotechnology building blocks. Refine self assembly processes into “Directed Techniques” that can provide nanostructures with short to long range order.

The Self Assembly Business IBM researchers have coaxed two different polymers – stringy molecules to assemble themselves into a honeycombed template of 20-nanometer holes. The honeycomb pattern, known as a diblock copolymer, was applied as a layer in the conventional process used to make transistors in a flash memory device, the thin cards used in digital cameras and handheld computers. NanoSonic, Inc., is refining a molecular self-assembly process, called electrostatic self-assembly (ESA), that produces thin films with material properties that can be precisely controlled. Nestle Foods and Proctor and Gamble is studying the application of self-assembly principles in food. NanoOpto Corporation is making optical components radically smaller, less expensive, denser, more integrated, and better performing by nano imprint pattern transfer.