Nanotechnology Lecture 4/12/04

1
Nanotechnology Lecture 4/12/04
Ways to create or manipulate nanostructures nanol
ithography dielectrophoresis (applications) opti
cal tweezers (applications) 2D-phase
separation Nanodevices nanowires 2D-photonic
crystals nanoscale optical sensors Ways to
study nanostructures atomic force
microscopy surface plasmon resonance
2
Nano-Patterned Surfaces
Two-dimensional phase separation can be used to
self-assemble a variety of microstructures at a
surface. Thin films of polymer solutions or
melts can become unstable and either de-wet the
surface or undergo phase separation. Three main
types of polymer films have received the most
attention homopolymers A-A-A-A-A-A-A-A-A b
lends of homopolymers A-A-A-A-A-A-A-A-A and
B-B-B-B-B-B-B-B-B block copolymers
(A-A-A-A-A-A)-(B-B-B-B-B-B-B)
3
Homopolymers
The basic physics is best illustrated for
homopolymers. Polymer solutions or melts are
spread on a surface by dipping or spin- coating
(allows precise thickness control). When the
spreading coefficient S is negative and the
Hamaker constant AS/F/V is positive, the film is
unstable and dewets via a spinodal
decomposition mechanism. The interplay
between dewetting (causing roughening of the
film surface) and capillary forces (favoring a
smooth surface) results in the film becoming
unstable on a characteristic dimension, proportion
al to the square of the original film thickness
and the square-root of the surface tension.
4
Spinodal decomposition leads to phase
separation on a characteristic length scale ? .
spontaneous surface fluctuations.
shorter wavelengths are opposed by surface
tension
h
h

x
x
longer wavelengths are un- favorable at high
instability
?
h
h
x
x
5
Spinodal-like Pattern Formation
Fluctuations in film thickness terminate in the
formation of holes in the film, eventually
leaving isolated droplets of polymer (AFM image).
6
Blends of In-compatible Homopolymer
Even slightly unfavorable enthalpic interactions
between constituents of a polymer blend will lead
to phase separation in 3D or 2D. In the absence
of preferential wetting of the surface by one of
the polymer components, 2D, lateral phase
separation occurs with the minority component
occurring as patches surrounded by majority
component. The patch size depends on the
viscosity and interfacial free energy between
phases, but is typically on the micron scale.
The polymer component more highly swollen with
solvent will shrink more as the film dries,
leaving a depression relative to the less swollen
component. As a result, patches appear as islands
or holes 100 nm high/deep.
PMMA
PVP
PS
substrate
7
In-compatible Homopolymer Blends (no preferential
wetting)
Upon increasing the fraction of minor component,
the holes/islands percolate to give a
bicontinuous pattern. Reversal of roles by the
minor/major components leads to holes reverting
to islands or vice- versa. The progression of
patches of one component increasing in number,
going through a bicontinuous morphology, and then
inverting to patches of the other component is
shown in these AFM images. Length scale is still
controlled by fluid viscosity/interfacial energy.
.
8
In-compatible Homopolymer Blends (with
preferential wetting)
When one component preferentially wets the
substrate, the symmetry is broken and a lamellar
phase structure develops - the wetting component
forming an enriched layer at the surface and an
adjacent depleted layer enriched in the other
component of the blend - via a process called
surface-directed spinodal decomposition.
wetting component
non-wetting
wetting component
substrate
unstable
substrate
9
In-compatible Homopolymer Blends (with
preferential wetting)
Ideally, the fractions of the components and the
overall film thickness would allow for the
preferentially wetting components to be at
the substrate and air interface. The central
lamellar layer can then be too thin and develop
fluctuations which grow via Rayleigh
instability. These capillary fluctuations pinch
off the central layer into droplets/ islands with
characteristic dimensions.
substrate
lateral phase separation
10
Internally Incompatible Blocks
Unfavorable interactions between A and B blocks
give local phase separation into distinct A -rich
and B-rich regions. Because blocks are attached
to one-another, the size and shape of these
regions depends on the absolute and relative
sizes of the blocks. Symmetric blocks (A/B of
the same size) phase separate into a lamellar
stacking of alternating A and B layers oriented
parallel to the substrate. The layer thickness
will scale with radius of gyration of the blocks
(?n). For asymmetric blocks (A gt B), the
minority block (B) is confined to first
cylindrical and then spherical aggregates. .
L
11
Incommensurability in Two-Dimensions
Refers to a mismatch between the applied film
thickness and the natural layer thickness -
determined by the block size. If the film is
not some multiple of the lamellae repeat
distance, parallel orientation is frustrated, so
that lamellar orientation develops running normal
to the substrate. Asymmetric di-blocks would
normally form hexagonally close-packed cylinders
oriented parallel to the surface. But if
the cylinder diameter exceeds the layer
thickness, then orientation is forced normal to
the substrate. .
12
Incompatible Blocks with Preferential Wetting
The lower surface tension block prefers the
air/film interface. If the other block can
reside at the substrate/film interface, then the
lamellae can orient parallel to the surface.
Commensurability between the layer thickness and
the inter-lamellar spacing controls the surface
topology. When the film can accommodate an exact
number of lamellar repeat units, the film will be
smooth and surface chemically homogeneous. When
the film thickness lies between one of these
periodic values , the extra increment in
thickness undergoes lateral phase separation into
islands consisting of full lamellar layers and
holes. The surface fraction of islands
increases as the increment in thickness
approaches that of an additional lamellar layer.
Thus islands, then bicontinuous, and then
dispersed holes occur.
13
Incompatible di-block copolymer with preferential
wetting.
Average excess (gtnL) film thickness between L/2
and L, forms extra layer of thickness L with
holes accounting for missing volume
Average excess film thickness between L/3 and
L/2, holes percolate to give bicontinuous
morphology
Average excess film thickness less than L/3,
breaks-up into islands of thickness L
14
Summary of 2-D Phase Separation in Films
Can be used to create periodic surface features
on micron length sales with height variations on
the order of 10-100 nm. Can be used to create
periodically inhomogeneous surface chemistry (in
terms of wettability or reactivity). Generally
requires a specially prepared substrate. The
majority of work has been done with organic
soluble polymers. Water-based systems need to be
developed. Applications to gas separating
membranes, photodiodes and photo- voltic devices,
light-emitting diodes, antireflective coatings,
and to increase the bio-compatibility of
implanted medical devices.
15
Dielectrophoresis - a way to move nano-particles
Defined as the lateral motion imparted on
uncharged particles as a result of polarization
induced by a non-uniform electric field.
How does it arise?
Particles suspended in a medium will generally
have a different dielectric permeability, ? ?
i?, than that medium. If so, then an applied
electric field will induce charges to appear at
the particle / medium boundary and the creation
of an electric dipole moment.
()
(-)
When ?P gt ?m, the particle is polarized more
easily than the medium by the external field and
the induced dipole moment is aligned with the
external field. The dipole moment ? (?q) d
can be quite large because the induced charge ?q
is separated by the particle dimension d (much
larger than a typical molecular dimension).
16
Dielectrophoresis - a way to move nano-particles
When ?P lt ?m, the particle is not as easily
polarized as the medium and the induced dipole
opposes the external field. As the frequency of
the applied field is increased, it eventually
becomes difficult to reverse the polarization
within the particle. However, the effects
persists even beyond 50 MHz.
Necessity of a non-uniform electric field
In a parallel plate capacitor (uniform electric
field) the total force on a dipole is zero F
(?-) E(0) (?) E(d) E (?- ?) 0
(?-)
(?)
d
17
Dielectrophoresis - necessity of non-uniform field
But in an electric field gradient, such as
is found with a pin and plate electrode set,
have E(0) gt E(d), so that F (?-) E(0) (?)
E(d) and the particle will migrate along a
field line towards (in this case) the higher
field of the pin electrode.
(?)
(?-)
()
(-)
Basis for separation of colloidal particles
Bacteria - gram-positive bacteria have cell walls
composed of open networks which are more
polarizable than gram-negative bacteria
(lipid/protein cell walls). So gram-positives
are attracted towards high fields,
gram-negatives repulsed. Viable / non-Viable
cells - non-viable cells have degraded
membranes with unregulated ion diffusivity (high
polarizable), while viable cell membranes
actively resist non-specific ion diffusion.
18
Application - Microwires wires of micron or
sub-micron cross-section
Dielectrophoresis can be used to self-assemble
microwires from colloidal nanoparticles suspended
in water. Hermanson et al. (Science 294 1082
(2001)) have applied an alternating voltage to a
suspension of gold nano- particles, 15-30 nm in
dimension.
Application of the field allows formation of
thin metallic fibers which span the gap, growing
out from each electrode at up to 50 ?m/s.
E
Planar electrodes 50-250 V AC 50-200 Hz E 250
V/cm
2 - 5 mm gap
Advantages of the process
Alternating voltage allows particle manipulation
without the complications of electro-osmotic or
electrochemical (dc) effects. Structures can be
assembled in situ, to create wet electronic
circuits
19
Application - Microwires
Controlling factors Field strength - must exceed
a threshold to overcome electrostatic repulsion
between particles. Nanoparticle concentration -
must exceed a threshold to encourage agglomeration
of particles. Electrolyte level - higher ionic
strengths enhance particle agglomeration
current (mA)
Self-repairing - at high applied voltage, the
wires burn out, only to spontaneously re-build
themselves by agglomeration of new nano- particles
time (s)
Performance
Simple ohmic behavior I? V with resistivity of
10-5 to 10-6 ohm m. Coated wires possible - grow
in mixed suspension of metallic particles
and polystyrene latex spheres - get metallic core
surrounded by shell of polymer Act as chemical
sensors - adsorption of trace agents from
solution affects resistivity because of high
wire surface area.
20
Application - 2D Photonic Crystals
Photonic crystals have a periodicity comparable
to the wavelength of light - used to manipulate
light on the microscale. They can be
self-assembled as a 2D colloidal crystal between
the gap of a gold planar electrode cell. Lumsdon
et al. (Appl. Phys. Lett. 82 949 (2003)) report
that the single crystal forms rapidly (lt 30 s)
and reversibly.
()
(-)
?
diffraction
hexagonal diffraction pattern
Charged polystyrene microspheres (0.5 ?m) form a
densely packed monolayer. The scattering angle ?
is controlled by the interparticle interaction
Separation in water gt 10-4 M NaCl gt 10-3 M NaCl
21
Nanoscale Optical Biosenors
Biosensors for the diagnosis and monitoring of
diseases, drug discovery, proteomics, and
environmental detection of biological agents are
in great demand. Fundamentally, a biosensor is
derived from the coupling of a ligand-receptor
binding reaction to a signal transducer. Optical
sensors based on evanescent electromagnetic
fields, particularly those based on propagating
surface plasmon polaritons (SPP) at planar gold
surfaces, are fast becoming the method of choice.
ligand
receptor
Ligand /receptor binding triggers a detectable
change in the receptor
22
Nanoscale Optical Biosenors
Surface plasmon resonance (SPR) can be used to
monitor a wide range of analyte-surface binding
interactions, such as the adsorption of
small molecules and ligand-receptor binding. The
sensing mechanism of SPR is based on the
measurement of small changes in the refractive
index that occur in response to the analyte
binding at or near the surface of noble metal
(Au, Ag, Cu) thin films.
Chemo/Bio Sensors based on SPR spectroscopy have
many advantages - refractive index sensitivity
of 1 part in 106, corresponding to an areal
mass sensitivity of 10-1 pg/mm2. - A sensing
length scale determined by the exponential decay
of the evanescent electromagnetic field 200
nm. - Multiple modes of detection angle shift,
wavelength shift, imaging - Real-time detection
on the 0.1-103 s time scale for measurement of
binding kinetics. - Lateral spatial resolution of
up to 10 microns.
23
Surface Plasma Resonance Spectroscopy
What it is - An electron charge density wave
phenomenon arising when light is reflected from
the surface of a metallic film under specific
conditions
The resonance is a result of energy and momentum
being transferred from incident photons to
surface plasmons - collective oscillations of
conduction free electrons in metals. It is very
sensitive to the refractive index of the medium
on the opposite side of the film from
which reflection occurs.
24
The Surface Plasma Resonance Effect
At the interface between two non-absorbing media
of refractive indices n1 and n2, with n2 lt n1
n2 (air) lt n1
n1 (glass)
?
Light incident at the interface from the medium
of higher index will undergo total internal
reflection if the incident angle is above a
critical angle. The light reflects back into
the higher index medium but leaks an electrical
field intensity called an evanescent field wave
(efw) into the low index medium.
25
The Role of the Metallic Film
The amplitude of the evanescent field wave
decreases exponentially with distance into the
lower index medium, with a penetration length on
the order of the wavelength of the incident
light
plasmon, ksp
n2 (air)
gold film
efw, kx
n1 (glass)
If the n2/n1 interface is coated with a
conducting material - like gold or silver, a
component of the evanescent field wave may
penetrate the metal layer and excite
electromagnetic surface plasmons
propagating within the conductor surface at the
air (n2) interface.
26
The Resonance Condition
The plasmon will enhance the evanescent wave,
enhancing its penetration depth. The magnitude
of the wavevector kx of the efw depends on the
local refractive index and the angle of incidence
as kx (2?/?) n1 sin ? The wavevector of the
surface plasmon wave, for a gold film (ngold)
and sample medium n2 , is ksp (2?/?) ?(ngold2
n22)/ (ngold2 n22) Thus as the incident angle
or the wavelength is varied, kx can cross ksp, at
which point the efw field wave will excite the
surface plasmon . As a result, there will be a
dip in the reflected intensity.
reflected intensity
?
27
SPR as a Sensitive Detector
The gold surface can be used as a detector for
adsorbable species in the medium n2, which can be
air or a solution. Adsorption of proteins, for
instance, from solution onto the gold surface
changes the local n2 (within the range of the
efw) and hence shifts the reflection angle
(or alternatively the wavelength) for the
resonance condition.
reflected intensity
shift in resonance angle upon adsorption
?
The greater the extent of adsorption, the greater
the change in n2 and the larger shift in angle.
Can sense femto-molar adsorption as well as
conformational changes in the adsorbed molecular
layer.
28
Fabrication of Triangular Silver Nanoparticles
Triangular silver nanoparticles can be fabricated
by nano-lithography. A stable gold colloid is
first prepared with a particle size of about 15
nm. A few micro-liters of this colloid is drop
coated onto a glass coverslip and dried to give a
single-layer colloidal crystal mask of Ag
nanoparticles (hexagonally close packed). A thin
gold film is then vapor deposited, contacting
the glass only in the gaps between the hexagonal
packing. The nanosphere mask is then removed by
sonication to leave triangular shaped silver
deposits of 50 nm height and 100 nm width.
Ag vapor
sonicate
Dimensions controlled by colloid particle size
and vapor deposition time.
29
Functionalization of Silver Triangles
The silver surface is covered with self-assembled
monolayers of 11- mercaptoundecanoic acid (-COOH
tipped) and/or 1-octanethiol (-CH3 tipped).
CH3
HOOC
CH3
biotin
SH
SH
SH
SH
glass prism
silver
The carboxyl moieties are reacted at low levels
with biotin to give a biotinylated silver
surface - about 100 sites per nanoparticle
30
Example of Nanoscale Affinity Biosensing
Streptavidin shows an extremely high binding
affinity to biotin (Ka 1013 M-1). Contacting
very dilute streptavidin solution with the
functionalized surface results in specific
binding, causing a subtle change in the local
refractive index of the medium surrounding the
nano-triangle
CH3
CH3
streptviden
SH
SH
SH
SH
glass prism
silver
The resonance condition for the nano-triangle
with bound streptavidin changes. Detect via
localized plasmon resonance as a shift in
wave- length of peak absorption.
31
Sensitivity of Nanosensor Array
The LSPR nanosensor operates by detecting
refractive index changes within the localized
electromagnetic fields surrounding the
nanoparticles. Although 100 nM strepavidin gives
a saturation wavelength shift of 27 nm (??max), 1
pM already gives a reproducible 4 nm red shift.
The reason for this non-linear behavior lies in
the nature of the binding constant. The relative
wavelength shift (?? /??max)measured versus a
range in streptavidin concentration (10-15 to
10-6 M) very nicely follows a Langmuir isotherm
with Ka 1011 M-1.
1.0
?? /??max
0.0
-15 -10 -5
log strepavidin
32
The Future Promises Greater Sensitivity
Currently can detect down to 1 pM for high
affinity binding. For high through- put
screening applications, ultimately aim for single
nano particle interrogation so that adjacent
nano-triangles can be sensitized for different
species. Complex mixtures could then be
completely and simultaneously analyzed for each
component with the limit of detection tending to
single molecules.
high affinity
low affinity
33
Typical Experimental Setup
detector
laser source
prism
?
metal film
adsorbed film
aqueous solution
34
Optical Tweezers - a manipulation tool in
nano-technology
Use light to manipulate microscopic objects in
the size range from a few nanometers up to about
a micron.
A strongly focused laser beam is used to catch
and hold dielectric particles. The use of
optical traps was first introduced by
scientists at Bell Laboratories in 1984.
specimen plane
How they work
laser
objective
optical trap
A laser is focused by a microscope objective to a
spot in the specimen plane. Usually, an infrared
laser is chosen to minimize sample damage.
35
Light possess momentum and can exert a pressure
All light consists of photons that each have a
momentum p, whose magnitude is ? p ? h/? and
whose direction is that of propagation. The
intensity of light is the number of photons
passing a given area A in unit time as given by
the Poynting vector S. The momentum flux from
light of this intensity is d d P/dt (n/c)
S dA The radiation pressure is the force exerted
per unit area on an object due to its
interaction with light. When light is reflected
from or refracted by an object, the momentum of
the light changes. The total force on the object
is the difference between the momentum flux
entering the object and that leaving the
object F (n/c) ?? (Sin - Sout) dA
36
Light exerts a force on objects it encounters
Reflection With normal incidence on a mirror,
Sin - Sout, F 2(n/c) ?? Sin dA Thus
objects are pushed by the reflection of light
from their surface. For 100 reflection from a 60
W light bulb, F 2(n/c) W 4 x 10-7 N ! Only
objects weighing less than 1 ?g can feel this
force.
Sin
Sout
37
Light exerts a force on objects it encounters
Refraction Any change in the direction of light
by refraction from an object will change the
momentum of light. The object must undergo an
equal and opposite momentum change.
pparticle
Plight
pparticle
Plight
polystyrene bead nbead (1.55) gt nmedium (1.33)
Pnet
So, objects are pulled towards the path of the
incident light by refraction!
38
In an optical tweezer - particles are pulled
towards the focus
The gaussian incident intensity of the source is
brighter in the center than at the edge. Thus the
refraction of light from the portion of
the particle nearest the center pulls strongly
towards the center of the beam while refraction
of the light from the edges of the source
pulls only weakly away from the center.
Problem reflective particles can be pushed out
of the trap. This tendency can be overcome by
using dual beam traps in which a particle is
illuminated from the front and back so that
the reflective pushes cancel out.
39
Whats it like inside an optical trap?
Dielectric (polystyrene) particles in a trap feel
a restoring force towards the center of the
focused laser. This restoring force
is proportional to the distance between the focal
point and the particle center - it behaves like
an optical spring.
x
Restoring force F ? x where ? is the
stiffness of the trap
So, measuring the displacement of the particle
(eg., by projecting the image of the bead onto a
quadrant photo-diode) determines the displacing
force acting on the particle (if ? is known).
Since x can be measured with an accuracy of 10
nm and ? is typically 50 pN/?m, a force
resolution of 0.5 pN is possible.
40
Measuring small forces with optical tweezers
Before quantitative measurements can be made, the
trap must be calibrated. This is usually done
in one of two ways Viscous drag The force due
to viscous drag on a dielectric sphere of radius
r is Fvis 6??rv , where v is the relative
velocity of the sphere in a medium of viscosity
?. Thus if the fluid surrounding a particle
fixed in a trap is driven at a fixed flow rate,
Fvis is known and the trap force constant ? can
be extracted from the particle displacement x.
flow velocity v
no flow
Fvis ?x
displacement x
0
0
41
Measuring small forces with optical tweezers
Brownian motion The Brownian motion of a
trapped sphere is described by the Langevin
differential equation 6??r dx/dt ? x
F(t) where F(t) is the stochastic force due to
thermal motion. Although this random force has
an average value of zero, the modulus of the net
displacement x is non-zero and can be used to
calibrate ?. Assuming that the trapping
potential is harmonic ( V(x) 1/2 ? x2), ?x2?
kBT/?
displacement ? x2?
0
0
42
Applications to Biochemistry
Attach a single DNA molecule to a polystyrene
bead coated with streptavidin (DNA labelled at
one end with biotin) and suspend the bead in an
optical trap. Molecular motors RNA polymerase
is an enzyme which copies DNA sequences to make a
single-stranded messenger RNA (mRNA) in the
process known as transcription. The mRNA is then
used by the ribosome to create a specific
protein. Energy is consumed in the process, by
which the polymerase moves along the DNA
strand. Thus RNA polymerases are molecular motors
- using energy to create motion and generating
forces in the cell. Optical tweezers can be used
to sense these forces.
43
Molecular Motors
RNA polymerase-bound bead is moved to stretch
DNA, exerting a force opposing transcription. At
high enough displacement (tension in DNA strand),
the molecular motor stalls! RNA polymerase is a
strong motor, exerting forces up to 25 pN.
mRNA
DNA
RNA polymerase
optically trapped bead
glass micro-pipette
move to stretch DNA
44
Unzipping DNA / RNA
RNA transcription and DNA replication require the
double-stranded DNA (ds-DNA) to be converted into
ss-DNA (splitting the double helix) - called the
helix-to-coil transition. Stretch a ds-DNA
molecule between two polystyrene beads
B
B
biotin
streptavidin-coated bead
45
Unzipping DNA / RNA
Stretching beyond a contour length of 0.34 nm /
base pair encounters a steeply rising force
curve. But allowing one end of the DNA to rotate
freely gives a cooperative over-stretching
transition at about 65 pN. Very little
additional force is necessary to stretch
the molecule to 1.7 times its contour length -
corresponding to a force- induced melting in
which the base pairs holding the two DNA
strands together break as the DNA unwinds.
100
ds-DNA
overstretching transition
force (pN)
ss-DNA
DNA extension per base pair (nm)
0
0.6
0.2
0.4