Materials in Nanotechnology
E SC 213
© 2013 The Pennsylvania State University
Unit 1
An Introduction to Colloidal
and Self-Assembled Materials
Lecture 2
Self-Assembly
© 2013 The Pennsylvania State University
Outline
• Review of Solutions
• Colloids and Colloidal Chemistry
• Self-Assembly
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Solutions
• A homogeneous mixture of two or more
substances
• Most common:
– Solid dissolved in liquid
– Mixture of two liquids
– Gas dissolved in liquid
• There are other types as well (solid-solid,
gas-solid, etc.)
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Solution Definitions
solvent molecule
Homogeneous
Solution
solute molecule
Solvent: The substance that dissolves the solute; usually the primary component
Solute: The compound (e.g., salt or sugar) that is dissolved in the solvent
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Properties of Solutions, Solutes, and Solvents
Solutions
• Concentration: reported in several ways
– Mass basis:
– Molar basis:
wt% =
mass of solute
mass of solute + mass of solvent
Molarity (M) =
Moles of Solute
Liters of Solution
• pH: a measure of the H+ concentration in
an aqueous solution.
– High pH = basic; low pH = acidic
– pH is a log scale, not linear
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Properties of Solutions, Solutes, and Solvents
Solutes
• Ionic: dissociate into cations and anions
when dissolved in a good solvent (usually
water)
– Examples: table salt (NaCl), NaOH
• Molecular: do not dissociate; remain as
whole molecules in solution
– Examples: sugar (sucrose), ethylene glycol
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Properties of Solutions, Solutes, and Solvents
Solvents
• Each solvent has its own set of properties
– melting point
– boiling point
– polarity
– density
– index of refraction
– possible impurities
– health and safety
Solvents are chosen for a particular
application based on these factors.
Goal: Find the best one for the job
while minimizing risks.
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Properties of Solutions, Solutes, and Solvents
• Rule of thumb: “Like dissolves like”
– This mainly applies to the polarity of solvents
vs. solutes.
• For example, which solvent is better suited
to cleaning vacuum grease off of a glass
fitting? water, acetone, hexanes
• Which pairs of liquids are miscible?
– water & ethanol, ethanol & IPA, methanol &
hexanes, acetone & water, hexanes &
pentane, water & pump oil
© 2013 The Pennsylvania State University
Solvent Polarity Index
More
Polar
Less
Polar
Solvent
Polarity Index
Normal bp(°C)
Water
9
100
DMSO
7.2
189
Acetic Acid
6.2
118
Ethanol
5.2
78
Methanol
5.1
65
Chloroform
4.1
61
Isopropanol
3.9
82
Methylene chloride
3.1
40
Diethyl ether
2.8
35
MTBE
2.5
55
Toluene
2.4
110
0
35,69,98
Pentane, hexane, heptane
© 2013 The Pennsylvania State University
Properties of Solutions, Solutes, and Solvents
Other properties of solvents that may be important
to particular applications:
– Halogenated or non-halogenated
– Aromatic vs. non-aromatic
– Hydrogen bonding capability
– Anhydrous (dry) or not (trace water)
– Volatility
– Residues left after evaporation
– Ability to be sublimed
– Stability: ethers decompose over time to form
explosive peroxides
© 2013 The Pennsylvania State University
Outline
• Review of Solutions
• Colloids and Colloidal Chemistry
– What is a Colloid?
– Types of Colloids and Examples
– Properties and Applications
• Self-Assembly
© 2013 The Pennsylvania State University
What is a Colloid?
The term colloidal refers to a state of
subdivision, implying that the molecules or
particles dispersed in a medium have at
least one dimension roughly between 1 nm
and 1 μm.
http://goldbook.iupac.org/
© 2013 The Pennsylvania State University
Comparative Size Scale
Proteins
HIV Virus
100 nm
White
Blood Cell
12-15 um
E. Coli
0.8-2 um
All of these could be
classified as colloidal
particles
Red
Blood Cell
6-8 um
Colloidal Particles
Nano-Scale
1 nm
10 nm
Meso-Scale
100 nm
1 μm
10 μm
100 μm
1000 nm = 1 μm
1 nm
5 nm
10 nm
100 nm
© 2013 The Pennsylvania State University
General properties of Colloids
• 2-phase systems: colloids
have a dispersed (internal)
phase and a continuous
(external) phase
• Large interfacial area between
the two phases, due to small
dimensions of the dispersed
phase
• Colloidal particles “are all
surface”
• Therefore, surface effects
dominate volume effects
© 2013 The Pennsylvania State University
Dispersed
Phase
Continuous
Phase
General properties of Colloids
• The particles are not molecularly dissolved
in the medium (solvent)
• Colloid ≠ solution
• When properly stabilized, the colloidal
particles do not aggregate or settle out
over time
• Colloids can be any combination of the
three states of matter, but the most
common colloidal mixtures consist of solid
(or liquid) particles suspended in a liquid
medium
© 2013 The Pennsylvania State University
Naming of Colloids
Dispersing
Medium
Solid
Solid
Solid
Liquid
Liquid
Liquid
Gas
Gas
Dispersed
Phase
Solid
Liquid
Gas
Solid
Liquid
Gas
Solid
Liquid
Colloid
Name
Solid sol
Gel
Solid foam
Sol
Emulsion
Foam
Solid aerosol
Aerosol
Paul Davies, School of Chemistry, University of Bristol
http://www.chm.bris.ac.uk/webprojects2002/pdavies/
© 2013 The Pennsylvania State University
Examples of Colloids
Dispersed
Phase
Continuous
Phase
Type
Example
Liquid
Gas
Aerosol
Fog, Hairspray
Liquid
Liquid
Emulsion
Salad Dressing
Liquid
Solid
Solid Emulsion
Pearl, Opal
Solid
Solid
Solid Suspension
Pigmented Plastics,
Stained Glass
Solid
Liquid
Sol or Paste
Ink, Toothpaste
Solid
Gas
Aerosol
Inhalers, Smoke
Gas
Liquid
Foam
Fire Extinguisher,
Soap Suds
Gas
Solid
Solid Foam
Pumice, Styrofoam
http://www.rsc.org/ chemistryworld/Issues/2003/February
© 2013 The Pennsylvania State University
Experiment: Finely Dividing a 1 cm3 Cube
Steps
1. Start with a cube 1 cm
on each side
2. Cut it into thin sheets
only 10 nm thick
3. Then cut each sheet into
10 sticks
4. Finally cut each stick into
nm sized cubes
Calculate
• Total number of pieces
arising from original cube
• Surface area of each
smaller piece
• Total surface area of all
pieces
• Note: volume remains
constant (1 cm3)
http://www.du.edu/~jcalvert/phys/colloid.htm
© 2013 The Pennsylvania State University
Experiment: Finely Dividing a 1 cm3 Cube
Starting
Volume
(Not Colloidal)
Laminated:
Colloid
Platelets
Fibrillar:
Colloidal
Fibers
Corpuscular:
Colloidal
Particles
1 x 1 x 1 cm
1 cm x 1 cm x 10 nm
1 cm x 10 nm x 10 nm
10 nm x 10 nm x 10 nm
1
106
1012
1018
Surface area
per piece (m2)
6 x 10-4
2 x 10-4
4 x 10-10
6 x 10-16
Total surface
area (m2)
6 x 10-4
200
400
600
Dimensions
Number of
pieces
SA of all pieces
needed to make
up original volume
Surface area to volume ratio increases as size of particle
decreases. Colloids are almost all surface area!
http://www.du.edu/~jcalvert/phys/colloid.htm
© 2013 The Pennsylvania State University
Properties of Colloids
The small size of colloidal particles lends
them interesting properties, including:
– They scatter light (solutions do not)
– The particles are subjected to Brownian
motion
– The surfaces of particles may become
charged, depending on the medium
– Charged colloidal particles can be moved
(separated) by an electric field (e.g.,
electrophoresis of DNA and proteins)
© 2013 The Pennsylvania State University
Scattering of Light by Colloidal Particles
Attenuation = Reduced intensity of light passing through a sample due to
absorption and scattering. Homogeneous solutions do not scatter light.
Colloidal suspensions do scatter light.
Initial
Beam
Intensity
(I0)
Final
Beam
Intensity
(I)
Light scattered
in all directions
Colloidal Particles
Homogeneous
Solution
Attenuation due only to
absorption of light by
molecularly dissolved
species (chromophores)
in the solution
Attenuation due to
scattering of incident
light by colloidal
particles. Absorption may
also occur
© 2013 The Pennsylvania State University
DLS: Dynamic Light Scattering
Laser
Relative Amount
• The scattering of light by colloidal particles can be put to good use
• Measurements of scattering intensity versus time can be correlated to the
Brownian motion of colloidal particles
• Mathematical analysis of the signal is used to calculate the speed of the
particles as they diffuse through the sample
• The speed is related to particle size: On average, small particles move faster
than larger ones
Colloidal Particles
Scattered
Light
Particle Size (nm)
Detector
© 2013 The Pennsylvania State University
http://www.malvern.com/
Colloidal Particles in Nanotechnology
Polymer molecules
dispersed in solvent
Proteins and DNA
in biotechnology
Dendrimer: starshaped polymer
Metal NP
Polymer nanospheres
Shell NP: hollow organic,
inorganic, or metal sphere
Liposome: hollow particle
made from lipids
© 2013 The Pennsylvania State University
Emulsion
Formation of Colloidal Particles
Physical Methods
• Grinding or milling
Condensation Methods
• Flame-spray
• Liquid phase synthesis
Roller Mill
Precursor Material
Ball Mill
Growing Particle
© 2013 The Pennsylvania State University
Example: Formation of Gold Nanoparticles
Sodium
Citrate
HAuCl4
Red Color = Gold NP
HAuCl4
Gold
NP
Heat
1. Heat a solution of chloroauric acid (HAuCl4) up to reflux (boiling). HAuCl4 is
a water soluble gold salt
2. Add trisodium citrate, which is a reducing agent
3. Continue stirring and heating for about 10 minutes
• During this time, the sodium citrate reduces the gold salt (Au3+) to
metallic gold (Au0)
• The neutral gold atoms aggregate into seed crystals
• The seed crystals continue to grow and eventually form gold
nanoparticles
http://mrsec.wisc.edu/Edetc/nanolab/gold/index.html
J. Chem. Ed. 2004, 81, 544A.
© 2013 The Pennsylvania State University
Example: Formation of Gold Nanoparticles
Reduction of gold ions: Au(III) + 3e- → Au(0)
Nucleation of Au(0) seed crystals:
Seed Crystal
10’s to 100’s of Atoms
Growth of nanoparticles:
Isotropic
Growth
Spherical
Nanoparticles
Surface capped
with citrate anions
Seed
Anisotropic
Growth
Nanorods
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Adding surfactant to growth solution
caps certain crystal faces and promotes
growth only in selected directions
Stabilization of Colloids
• Remember: An important aspect of colloidal
engineering is the suspension of the particle in a
medium – often water
• Colloidal particles can be hydrophobic or hydrophilic.
• Hydrophilic groups generally contain oxygen and
nitrogen. They are water loving
• Hydrophobic colloids can be prepared in water only if
they are stabilized in some way. The lack of affinity
for water will cause them to settle or float
• More general terms are lyophilic (likes the external
phase) and lyophobic (dislikes the external phase).
These terms are used when the medium is not water
© 2013 The Pennsylvania State University
Stabilization of Colloids
How do the particles remain suspended in solution?
• For such small particles, the forces of Brownian motion exceed the force of
gravity, which otherwise would cause the particles to settle out
• Particles suspended in water often acquire a negative surface charge. Particles
with charged surfaces repel each other at short distances
• Steric repulsion can also be used to keep particles from aggregating. This is
useful for suspending neutral particles in non-polar continuous phases
Electrostatic Repulsion
+
+
+
-
+
+
- +
+
- -
- -
-
-
+
+
+
+
-
+
+
+
+
-
- --- -
-
+
+
- -
+
-
-
Steric Repulsion
+
+
+
+
+
+
+
+
+
+
© 2013 The Pennsylvania State University
+
+
Separating Colloids: Electrophoresis
Colloidal particles commonly take on a negative surface charge when dispersed in
water. In the presence of an electric field, the particles move through the
surrounding medium. Smaller particles move faster than larger particles. Over time,
the colloidal particles become separated according to size
Applications: DNA fingerprinting and protein isolation/purification
++++++++++++++
++++++++++++++
++++++++++++++
smaller
particles
Power
Supply
--------------Sample
1
Sample
2
Sample
3
Samples added to gel and
voltage applied
--------------Sample
1
Sample
2
Sample
3
--------------Sample
1
Negatively charged particles
migrate slowly towards the (+)
electrode
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Sample
2
larger
particles
Sample
3
End point: Particles
separated by size
Purifying Colloids: Dialysis
Application: Removing impurities from a colloidal suspension. Impurities could be
unreacted starting materials, by-products from particle synthesis, salts, excess
capping agents, etc
Dialysis Bag
x
x
x
x
x
x
x
x
x
x
x
x
x
x
Semi-permeable membrane allows small molecules to pass. Colloidal particles
are too large to pass through the membrane pores. Continuous rinsing with fresh
solvent eventually removes all unwanted small molecule impurities
© 2013 The Pennsylvania State University
Outline
• Review of Solutions
• Colloids and Colloid Chemistry
• Self-Assembly
– What is it?
– Forces and interactions
– Examples from nature
– Nanotechnology examples
© 2013 The Pennsylvania State University
What is Self-Assembly?
• Ever-evolving definitions
• Sometimes called self-organization
• One possible definition: a reversible
process that involves pre-existing, distinct
components of an initially disordered
structure
• Therefore, self-assembly ≠ formation
“Self-Assembly at All Scales,” G.M. Whitesides and B. Grzybowski, Science 2002, 295 (5564), 2418.
© 2013 The Pennsylvania State University
More on Self-Assembly
• Has origins in organic chemistry: structures are
determined bond-by-bond, but the structures are
molecules (less than about 0.5 nm in size)
• However, it is impossible to direct the formation (bondby-bond) of larger nano- and micro-scale structures
• Lithography is very useful for building larger structures
(~100 nm to microns), but is inherently a 2-D process. 3D structures have to be built layer-by-layer
• Self-assembly fills the processing gap by utilizing
specific (usually weak) interactions between molecules
to build 2-D and 3-D structures in the 10’s to 100’s nm
size range
“Self-Assembly at All Scales,” G.M. Whitesides and
B. Grzybowski, Science 2002, 295 (5564), 2418.
© 2013 The Pennsylvania State University
More on Self-Assembly
• It is not a “brute-force” technique
• You don’t get it by purchasing a tool from
a manufacturer
• In fact, it usually requires no tools at all!
© 2013 The Pennsylvania State University
Many things (living and non-living) spontaneously
organize over many length scales: Å to light year.
Å
nm
10-9
10-7
10-5
mm
10-3
10-1
101
km
103
105
107
109
AU
1011
1
ly
1013
1015
1E-10-10
1E-09 1E-08-8
1E-07 1E-06-6
1E-05 1E-04-4
1E-03 1E-02-2
1E-01 1E+00-0
1E+01 1E+0221E+03 1E+0441E+05 1E+0661E+07 1E+0881E+09 1E+1010
1E+11 1E+1212
1E+13 1E+1414
1E+15 1E+1616
10
10
10
10
10
10
10
10
10
© 2013 The Pennsylvania State University
10
10
10
10
10
Forces at Work in self assembly
Type or Scale of Self-Assembly
Molecular
Nano & Meso Scale
Macro-scale
van der Waals
Brownian Motion
Gravitation
Electrostatics
Capillary Forces
Electromagnetic Fields
Hydrogen Bonds
Entropic Interactions
Magnetic Interactions
Coordination Bonds
Self-assembly usually occurs in a fluid-like state. The materials (molecules or
particles) have to be able to move around. They sample many different
orientations and interactions with respect to each other. One orientation
(interaction) tends to be more favorable than others. Given enough time, the
structural elements optimize their local environments to produce a selforganized structure over a large volume
© 2013 The Pennsylvania State University
Bonding and Forces Revisited
• Covalent Bonds
Cl Cl
• Ionic (electrostatic)
-
+
Z  Z e2
F
r2
• Hydrogen Bonding
• van der Waals Forces
• Hydrophobic Interactions
© 2013 The Pennsylvania State University
F
1
r6
Competition Between Forces
Often, self assembly occurs due to a competition between two types of forces
or interactions. Each type of force acts over a characteristic length scale. The
table below illustrates the types of self-assembly arising from the interplay of
long range repulsions and short range attractions
Long-Range
Repulsion
Short-Range
Attraction
Example of SelfOrganized System
Hydrophobic/Hydrophilic
Covalent Bonding
Micelles, Lyotropic LC
Incompatibility/Insolubility
Covalent Bonding
Block Copolymers
Coulombic Repulsion
Electroneutrality
Ionic Crystals
Excluded Volume
Minimum Space Required
Thermotropic LC
Electric Field
Electric Dipole Interaction
Ferroelectric Domains
Magnetic Field
Magnetic Dipole Interaction Magnetic Domains
“From Self-Organizing Polymers to Nanohybrid and
Biomaterials,” S. Förster and T. Plantenberg,
Angew. Chem. Int. Ed. 2002, 41(5), 688.
© 2013 The Pennsylvania State University
Examples and Applications
• Simple case: crystallization of a compound from
solution
• For example: crystals of sugar forming when a
heated sugar solution is cooled
• By definition, a crystal is an ordered
arrangement of components. In this case, the
sugar crystals are comprised of highly ordered
sugar molecules
• Each sugar molecule develops specific contacts
with neighboring molecules in the growing
crystal
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Example: Molecular Crystals
solvent molecule
(water)
solute molecule
(sugar)
growing crystal
specific
intermolecular
interactions
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Inspiration from Nature
More complicated examples that show the power
of self-assembly:
• DNA double helix
– Consists of 2 strands of DNA
– Each strand contains base pairs covalently bonded to
a phosphate backbone
– The 2 strands are held together by hydrogen bonding
between complementary base pairs
• Protein folding
– Proteins are polymers of amino acids
– They fold into intricate 3-D structures
– This is discussed on the following slides
© 2013 The Pennsylvania State University
Example: Self-Assembly in Nature
Data storage on the
molecular level
DNA
•
•
•
•
•
•
•
Transcription
RNA
Translation
(Ribosome)
Protein
DNA sequence is the code for protein synthesis
Codon: 3 adjacent base pairs of DNA that code for one amino acid in the
resulting protein
Side-Chain
20 possible amino acids: each has a different side-chain
The side-chains have different chemical properties:
 large vs. small size (vdW volume and surface area)
 hyrdophobic vs. hydrophilic
 acidic vs. basic
 hydrogen bond donors/acceptors
 ability to form disulfide bonds (Cysteine: R-SH + HS-R  R-S-S-R)
 ability to coordinate to metal atoms
Side-chains help guide the protein to fold into a particular structure
Proteins must be folded properly in order to perform their functions
Some diseases arise from mis-folded proteins
© 2013 The Pennsylvania State University
Example: Self-Assembly in Nature
The terminology of protein folding
Primary Structure: The order of amino acids
that make up the protein. They are attached,
via covalent bonds, into a polymer chain
Secondary Structure: Folding of the
backbone chain of the protein into sheetlike or helical structures. This occurs to
satisfy the hydrogen bonding capabilities of
the back bone amide linkages
Tertiary Structure: Packing together of
secondary structure elements to form a
functional protein. Hydrophobic side-chains
generally pack to the inside of the protein,
while hydrophilic side-chains remain on the
solvent-exposed surface of the protein.
The solvent here is water
© 2013 The Pennsylvania State University
…-Gly-Ala-Tyr-…
amino acids bearing
different side-chains
Nanotechnology Applications
• The mechanisms involved in protein
folding are still being explored by
molecular biologists
• But, not all examples of self-assembly are
so complex
• In fact, scientists use much simpler
versions of self-assembly all the time
• Many applications have found their way
into nanotech processes and devices
© 2013 The Pennsylvania State University
Applications of Self-Assembly
Self-Organizing System
Application
Atomic, ionic, and molecular
crystals
Materials, optoelectronics
Self-assembled monolayers
(SAMs)
Microfabrication, sensors, nanoelectronics
Lipid bilayers and lipid films
Biomembranes, emulsions,
liposomes for drug delivery
Phase-separated and ionic layered
polymers
Nano-structured templates
Liquid crystals
Displays and TVs
Colloidal crystals
Nanosphere lithography, photonic
band gap materials
“Self-Assembly at All Scales,” G.M. Whitesides and B. Grzybowski,
Science 2002, 295 (5564), 2418.
© 2013 The Pennsylvania State University
Example: Self-Assembled Monolayers
• Self assembled monolayers (SAMs) refer to the
organization of extremely thin films (one molecule thick)
on solid surfaces
• Common examples: thiols on gold; silanes on oxide
surfaces
• The molecules that form SAMs are like surfactants. They
have two distinct regions: One part is attracted to the
surface; the other is not
• The molecules are sometimes called “Ligands,”
especially when talking about SAMs on metal surfaces.
• Applications: improved PDMS mold release; altered
hydrophobicity of surfaces
© 2013 The Pennsylvania State University
Example: Self-Assembled Monolayer (SAM)
Octadecanethiol: C18H37-SH
Surface Active Molecules
van der Waals
Forces
Specific
Interactions
Metal Surface
Specific example: alkane thiols on gold. These molecules have a long
greasy tail and an –SH head group. The –SH is attracted to the gold while
the hydrocarbon tail is exposed to the solvent or air
© 2013 The Pennsylvania State University
SAMs: Choosing the Best Ligand
• Ligands have to be chosen so that they
will bind and assemble on a surface
• Some ligands work on many surfaces
• Others work best only on selected
surfaces
• If a ligand does not bind well to a surface,
then it can be easily rinsed off and a SAM
does not form
• When designed properly, SAMs are
durable surface treatments
© 2013 The Pennsylvania State University
Ligands for Various Surfaces
Alkylated Ligand
Isonitrile
R-NC
Carboxylic Acid
R-COOH
Phosphonic Acid
R-PO(OH)2
Alcohol
R-OH
Amine
R-NH2
Amide
R-CONH2












Au






Zn






 = Yes SAM
 = No SAM
Surface
Thiol
R-SH
Sulfide
R-S-R
Phosphine
R3P
Cr, Ni, Fe, Al


Pt

Cu, Ag

Other Surfaces
Metal & Silicon Oxides
 Silanes R-SiCl3
Adsorption generally follows “Hard-Soft-Acid-Base” rules: Carboxylic and phosphonic acids adsorb onto
any metal oxide surface. Thiols and isonitriles adsorb onto soft metals (Cu, Ag, Au, Pt). However, phosphines
(which are considered soft) form monolayers on many surfaces (soft or hard). This could be due to their
oxidation to phosphine oxides
Pure and Applied Chemistry 1991, 63(6), 821-828.
© 2013 The Pennsylvania State University
Application: Surface Modification
OH
OH
OH
SAM
+
Hydrophobic Surface
Hydroxyl-Rich Surface
(Hydrophilic)
Reacts with hydroxyls
Water Drop
Contact Angle and Wettability
Hydrophilic
<30°
Hydrophobic
>90°
© 2013 The Pennsylvania State University
Super Hydrophobic
>150°
Application: Microcontact Printing
Liquid PDMS
Precursor
Cured PDMS Stamp
Master
Thiol
Coating
Gold Film
Substrate
Thiol SAM
Gold
Public Domain: Image Generated by CNEU Staff for free use
© 2013 The Pennsylvania State University
Example: Block Copolymers
• Block copolymers are similar to surfactants in that they
have two chemically dissimilar parts
• When cast into films, the two blocks do not want to mix
with each other (like oil and water)
• But the blocks are covalently attached to each other, so
they cannot move too far apart
• This causes a phase separation on the nanoscale.
• Nanoscale domains form a pattern that is characteristic
of the block copolymer composition (spheres, cylinders,
lamellae)
• Changing the ratio (1:2) changes the pattern
One Block Copolymer Molecule
Block 1
Block 2
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Example: Block Copolymers
Increasing Length of A 
One block
copolymer molecule
 Increasing Length of B
20:80
30:70
50:50
70:30
80:20
Spheres of A
Cyl of A
Lamellae
Cyl of B
Spheres of B
Structures form due to
phase-separation on the
nano-scale
Minimization of surface
area between two
incompatible phases
© 2013 The Pennsylvania State University
Example Process for Block Copolymers
The exact process route depends on the properties of the block copolymer being used to
create the pattern. This example is for PS-b-PMMA, which self-assembles when heated
to about 175 °C (above the Tg of both blocks, but not totally melted)
• Clean the substrate to remove any contamination
• Neutralize (randomize) the surface so that both blocks (PS and PMMA) have an
equal affinity for the surface. In other words, neither block will preferentially wet or
stick to the substrate surface
• Dissolve the polymer in a solvent (toluene or PGMEA) to make a dilute solution of the
polymer (1 wt% works well)
• Spin coat a thin film (35 nm) of PS-b-PMMA onto the randomized substrate
• Anneal (heat) the sample in a 175 °C vacuum oven for approximately 24 hours
• Cool the sample back to rt and verify the self-assembled pattern via FESEM or AFM
PS Block
PMMA Block
http://www.internano.org
© 2013 The Pennsylvania State University
Images of Block Copolymers
50:50 PMMA to PS
Lamellae
30:70 PMMA to PS
Cylinders of PMMA in a matrix of PS
100 nm
Increasing size of PMMA block
70:30 PMMA to PS
Cylinders of PS in a matrix of PMMA
Decreasing size of PS block
Overall size (PS + PMMA) remains constant
Light gray: PS
Dark gray: PMMA
Public Domain: Image Generated by CNEU Staff for free use
© 2013 The Pennsylvania State University
Surfactants
• Soaps and detergents are common
examples of surfactants
• Each molecule has a hydrophobic
tail and hydrophilic head
• When dissolved in water,
surfactants self-assemble into
micelles to minimize interactions
between the hydrophobic tails and
the water
hydrophobic tail
hydrophilic head
Surfactants can be classified
according to their head group:
• Anionic – negative charge
• Cationic – positive charge
• Non-ionic – no charge
• Zwitterionic – both (+) and (-)
The choice of surfactant depends on the application:
• Common soaps: anionic (good cleansing and high foaming)
• Baby shampoo: zwitterionic (mildness)
• Laundry detergent: non-ionic (lower foam and less sensitive to hardness ions)
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Example: Micelle and Reverse Micelle
Polar Solvent (Water)
Non-Polar Solvent (Oil)
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Example: Lipid Bilayers
Polar Head
Group
Non-polar Tails
Lipids are natural surfactants that selfassemble into cell membranes
Aqueous Extracellular
Environment
Non-polar
Membrane Interior
Aqueous Cytoplasm
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Liquid Crystals
• Liquid crystals exist somewhere between
a solid crystal (very ordered) and an
isotropic liquid (no order)
– Kind of like an organized liquid
– The molecules arrange themselves so that
they stack into a highly ordered pattern
– But the material is still fluid-like
• Phase change = transition from one state
(arrangement of molecules) to another
state
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Liquid Crystals
• Two general types of liquid crystals (LC):
– Thermotropic LC: change phase as
temperature increases/decreases
– Lyotropic LC: phase changes induced by
addition of solvent (change in concentration)
• Some liquid crystals have an interesting
property: they rotate the plane of
polarization of light. This property is used
in some LCD displays
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Example: Liquid Crystals (LC)
Thermotropic LC: change phase as temperature increases/decreases
Lyotropic LC: phase changes induced by addition of solvent (change in concentration)
Example of a liquid
crystalline phase
(there are many)
Heat
Liquid Crystal
Long range order but still
behaves like a fluid
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Isotropic liquid
no long range order
Application: TFT LCD
Backlight
Unpolarized Light
Polarizer 1
Polarized Light
Self-Assembled
Liquid Crystal
No Voltage
Applied
Voltage
Applied
Polarizer 2
 Light Passes
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 Light Blocked
Example: Colloidal Crystals
Similar to atomic and molecular crystals, except that the size of the
building blocks are much larger and the forces holding them together
are much weaker (compared to the size of the particles)
Colloidal: encompasses particles that have at least one characteristic
dimension in the 1 nm to 1 um range – small enough to be the subjects
of Brownian motion
Spherical
Colloidal Crystal
Colloidal
Particle
Closest-Packed Colloidal Spheres
HIV Virus
100 nm
Proteins
E. Coli
0.8-2 um
White
Blood Cell
12-15 um
Red
Blood Cell
6-8 um
Colloidal Particles
Nano-Scale
1 nm
10 nm
Meso-Scale
100 nm
1 μm
10 μm
100 μm
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Application: Nanosphere Lithography
Colloidal Solution of Nanospheres
Solvent
Evaporation
Drop Cast
Self-Assembled
Nanospheres
Substrate
Side View
Deposit
Metal
Dissolve
Spheres
Top View
Feature spacing can be tailored by
adjusting size of colloidal spheres
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Summary
• Colloid: refers to small size of objects;
encompasses nano- and meso-scale
• Colloids scatter light; homogeneous solutions do
not scatter light
• There are many everyday examples of selfassembly: soaps, proteins, LCDs
• Emerging nanotech applications rely heavily on
self-assembling molecules, polymers, and
particles
• Advances in self-assembly may eventually lead
to true “bottom-up” manufacturing
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