M. Meyyappan NASA Ames Research Center

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M. Meyyappan

NASA Ames Research Center

Moffett Field, CA 94035


email: mmeyyappan@mail.arc.nasa.gov

web: http://www.ipt.arc.nasa.gov

Nanometer



One billionth (10
-
9
) of a meter



Hydrogen atom 0.04 nm



Proteins ~ 1
-
20 nm



Feature size of computer chips 90 nm


(in 2005)



Diameter of human hair ~ 10 µm


Nanotechnology is the creation of
USEFUL/FUNCTIONAL

materials, devices and systems (
of any useful size
) through
control/manipulation of matter on the nanometer length scale and
exploitation of novel phenomena and properties which arise because
of the nanometer length scale:



Physical



Chemical



Electrical



Mechanical



Optical



Magnetic







Research and technology development aimed
to understand and control matter at
dimensions of approximately 1
-

100
nanometer


the nanoscale


Ability to understand, create, and use
structures, devices and systems that have
fundamentally new properties and functions
because of their nanoscale structure


Ability to image, measure, model, and
manipulate matter on the nanoscale to exploit
those properties and functions


Ability to integrate those properties and
functions into systems spanning from nano
-

to
macro
-
scopic scales

Corral of Fe Atoms


D. Eigler

Nanoarea Electron Diffraction
of DW Carbon Nanotube


Zuo, et.al

What Is Nanotechnology?

Source: Clayton Teague, NNI

(Definition from the NNI)



Examples


-

Carbon Nanotubes


-

Proteins, DNA


-

Single electron transistors




Not just size reduction but phenomena


intrinsic to nanoscale


-

Size confinement


-

Dominance of interfacial phenomena


-

Quantum mechanics




New behavior at nanoscale is not



necessarily predictable from what we


know at macroscales.

AFM Image of DNA


Quantum size effects result in unique mechanical, electronic, photonic,
and magnetic properties of nanoscale materials


Chemical reactivity of nanoscale materials greatly different from more
macroscopic form, e.g., gold


Vastly increased surface area per unit mass, e.g., upwards of 1000 m
2

per gram


New chemical forms of common chemical elements, e.g., fullerenes,
nanotubes of carbon, titanium oxide, zinc oxide, other layered
compounds

Unique Properties of Nanoscale Materials

Source: Clayton Teague, NNI



Atoms and molecules are generally less than a nm and we study


them in chemistry. Condensed matter physics deals with solids


with infinite array of bound atoms. Nanoscience deals with the


in
-
between meso
-
world




Quantum chemistry does not apply (although fundamental laws


hold) and the systems are not large enough for classical laws of


physics




Size
-
dependent properties




Surface to volume ratio



-

A 3 nm iron particle has 50% atoms on the surface



-

A 10 nm particle 20% on the surface



-

A 30 nm particle only 5% on the surface

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SECTION

II



Many existing technologies already depend on nanoscale materials


and processes



-

photography, catalysts are “old” examples



-

developed empirically decades ago




In existing technologies using nanomaterials/processes, role of


nanoscale phenomena not understood until recently; serendipitous


discoveries



-

with understanding comes opportunities for improvement




Ability to design more complex systems in the future is ahead



-

designer material that is hard and strong but low weight



-

self
-
healing materials



Recently, there has been an explosion of research

on the nanoscale behavior


-

Nanostructures through sub
-
micron self



assembly creating entities from “bottom
-
up”


instead of “top
-
down”


-

Characterization and applications


-

Highly sophisticated computer simulations to


enhance understanding as well as create



‘designer materials’



1959 Feynman Lecture
“There is Plenty of Room at the

Bottom”
provided the vision of exciting new discoveries if

one could fabricate materials/devices at the atomic/molecular

scale.




Emergence of instruments in the 1980s; STM, AFM


providing the “eyes”, “fingers” for nanoscale manipulation,

measurement…

STM

Image of Highly Oriented
Pyrolitic Graphite



Cluster



-

A collection of units (atoms or reactive molecules) of up to




about 50 units



Colloids



-

A stable liquid phase containing particles in the 1
-
1000 nm




range. A colloid particle is one such 1
-
1000 nm particle.



Nanoparticle



-

A solid particle in the 1
-
100 nm range that could be





noncrystalline, an aggregate of crystallites or a single




crystallite



Nanocrystal



-

A solid particle that is a single crystal in the nanometer range

Source: Nanoscale Materials in Chemistry, Ed. K.J. Klabunde, Wiley, 2001



Spherical iron nanocrystals




J. Phys. Chem. 1996,


Vol. 100, p. 12142

For example, 5 cubic centimeters


about 1.7 cm per side


of
material divided 24 times will produce 1 nanometer cubes and
spread in a single layer could cover a football field

Repeat 24 times

Nanoscale = High Ratio of Surface Area to Vol.

Source: Clayton Teague, NNI



In materials where strong chemical bonding is present, delocalization of valence

electrons can be extensive. The extent of

delocalization can vary with the size

of the system.




Structure also changes with size.




The above two changes can lead to different physical and chemical


properties,
depending on size



-

Optical properties



-

Bandgap



-

Melting point



-

Specific heat



-

Surface reactivity



-




-



Even when such nanoparticles are consolidated into macroscale solids, new


properties of bulk materials are possible.



-

Example: enhanced plasticity



For semiconductors such as ZnO, CdS, and Si, the bandgap


changes with size



-

Bandgap is the energy needed to promote an electron




from the valence band to the conduction band



-

When the bandgaps lie in the visible spectrum, a change




in bandgap with size means a change in color




For magnetic materials such as Fe, Co, Ni, Fe
3
O
4
, etc., magnetic

properties are size dependent



-

The ‘coercive force’ (or magnetic memory) needed to




reverse an internal magnetic field within the particle is




size dependent



-

The strength of a particle’s internal magnetic field can be



size dependent



In a classical sense, color is caused by the partial absorption of


light by electrons in matter, resulting in the visibility of the


complementary part of the light




On most smooth metal surfaces, light is totally reflected by the


high density of electrons no color, just a mirror
-
like


appearance.




Small particles absorb, leading to some color. This is a size


dependent property.


Example
: Gold, which readily forms nanoparticles but not easily

oxidized, exhibits different colors depending on particle size.



-

Gold colloids have been used to color glasses since early




days of glass making. Ruby
-
glass contains finely dispersed



gold
-
colloids.



-

Silver and copper also give attractive colors



C = ∆Q/m∆
T; the amount of heat
∆Q required to raise the


temperature by ∆T of a sample of mass m




J/kg ∙K or cal/g ∙K; 1 calorie is the heat needed to raise the temp. of

1 g of water by 1 degree.



Specific heat of polycrystalline materials given by Dulong
-
Petit law



-

C of solids at room temp. (in J/kg ∙k) differ widely from one to


another; but the molar values (in J/moles ∙k) are nearly the



same, approaching 26 J/mol ∙K; C
v

= 3 Rg/M



where M is molecular weight



C
v

of nanocrystalline materials are higher than their bulk


counterparts. Example:



-

Pd: 48%


from 25 to 37 J/mol.K at 250 K for 6 nm crystalline



-

Cu: 8.3%


from 24 to 26 J/mol.K at 250 K for 8 nm



-

Ru: 22%


from 23 to 28 J/mol.K at 250 K for 6 nm

The melting point of gold particles decreases dramatically as
the particle size gets below 5 nm

Source: Nanoscale Materials in Chemistry, Wiley, 2001



Start from an energy balance; assume the change in internal energy


(∆U) and change in entropy per unit mass during melting are


independent of temperature






= Deviation of melting point from the bulk value

T
o


= Bulk melting point




= Surface tension coefficient for a liquid
-
solid interface




= Particle density

r


= Particle radius

L


= Latent heat of fusion



Lowering of the melting point is proportional to 1/r






can be as large as couple of hundred degrees when the


particle size gets below 10 nm!




Most of the time,


the surface tension coefficient is unknown;


by measuring the melting point as a function of radius,


can be


estimated.




Note: For nanoparticles embedded in a matrix, melting point may

be lower or higher, depending on the strength of the interaction


between the particle and matrix.



For metals, conductivity is based on their band structure. If the


conduction band is only partially occupied by electrons, they can


move in all directions without resistance (provided there is a perfect

metallic crystal lattice). They are not scattered by the regular


building blocks, due to the wave character of the electrons.

v

= electron speed


o

= dielectric constant in vacuum




, mean time between collisions, is

/
v




For Cu,
v

= 1.6 x 10
6

m/s at room temp.;


= 43 nm,


= 2.7 x 10
-
14
s




Scattering mechanisms


(1)

By lattice defects (foreign atoms, vacancies, interstitial




positions, grain boundaries, dislocations, stacking disorders)


(2)

Scattering at thermal vibration of the lattice (phonons)




Item (1) is more or less independent of temperature while item #2

is independent of lattice defects, but dependent on temperature.




Electric current collective motion of electrons; in a bulk metal,







Ohm’s law: V = RI




Band structure begins to change when metal particles become


small. Discrete energy levels begin to dominate, and Ohm’s law is

no longer valid.



If a bulk metal is made thinner and thinner, until the electrons can

move only in two dimensions (instead of 3), then it is “2D quantum

confinement.”




Next level is ‘quantum wire




Ultimately ‘quantum dot’

Source: Nanoscale Materials in Chemistry, Wiley, 2001



Adsorption is like absorption except the adsorbed material is held near the surface

rather than inside




In bulk solids, all molecules are surrounded by and bound to neighboring atoms

and the forces are in balance. Surface atoms are bound only on one side, leaving

unbalanced atomic and molecular forces on the surface. These forces attract gases

and molecules


Van der Waals force,


physical adsorption or physisorption




At high temperatures, unbalanced surface forces may be satisfied by electron


sharing or valence bonding with gas atoms


chemical adsorption or


chemisorption



-

Basis for heterogeneous catalysis (key to production of fertilizers,




pharmaceuticals, synthetic fibers, solvents, surfactants, gasoline, other



fuels, automobile catalytic converters…)



-

High specific surface area (area per unit mass)



Frequently encountered powders:



-

Cement, fertilizer, face powder, table salt, sugar, detergents, coffee




creamer, baking soda…




Some products in which powder incorporation is not obvious



-

Paint, tooth paste, lipstick, mascara, chewing gum, magnetic recording




media, slick magazine covers, floor coverings, automobile tires…




For most applications, there is an optimum particle size



-

Taste of peanut butter is affected by particle size



-

Extremely fine amorphous silica is added to control the ketchup flow



-

Medical tablets dissolve in our system at a rate controlled by particle size



-

Pigment size controls the saturation and brilliance of paints



-

Effectiveness of odor removers is controlled by the surface area of




adsorbents.

From: Analytical methods in Fine Particle Technology, Webb and Orr



Adding certain inorganic clays to rubber dramatically improves


the lifetime and wear
-
characteristics of tires.


Why ?



The nanoscale clay particles bind to the ends of the polymer


molecules
-

which you can think of as molecular strings
-

and


prevent them from unraveling.

CNT is a tubular form of carbon with diameter as small as 1 nm.

Length: few nm to microns.


CNT is configurationally equivalent to a single or mutliple two
dimensional graphene sheet(s) rolled into a tube (single wall vs.
multiwalled).

CNT exhibits extraordinary mechanical
properties: Young’s modulus over

1 Tera Pascal, as stiff as diamond, and tensile
strength ~ 200 GPa.


CNT can be metallic or semiconducting,
depending on (m
-
n)/3 is an integer (metallic)

or not (semiconductor).

See textbook on

Carbon Nanotubes:
Science and
Applications,

M. Meyyappan,
CRC Press, 2004.



The strongest and most flexible molecular


material because of C
-
C covalent bonding


and seamless hexagonal network architecture




Young’s modulus of over 1 TPa vs 70 GPa for


Aluminum, 700 GPa for C
-
fiber



-

strength to weight ratio 500 time > for Al;



similar improvements over steel and




titanium; one order of magnitude




improvement over graphite/epoxy




Maximum strain 10%; much higher than any


material




Thermal conductivity ~ 3000 W/mK in the axial

direction with small values in the radial direction

See http://www.ipt.arc.nasa.gov/gallery.html for videos of bending, compression, etc., of CNTs



Electrical conductivity higher than copper




Can be metallic or semiconducting depending on chirality




-

‘tunable’ bandgap




-

electronic properties can be tailored through application of external






magnetic field, application of mechanical deformation…




Very high current carrying capacity


(10
7

-

10
9

A/cm
2
)




Excellent field emitter; high aspect ratio


and small tip radius of curvature are


ideal for field emission




Other chemical groups can be attached


to the tip or sidewall (called ‘functionalization’)

Sensors, Bio, NEMS




CNT based microscopy: AFM, STM…




Nanotube sensors: bio, chemical…




Molecular gears, motors, actuators




Batteries (Li storage), Fuel Cells, H
2
storage




Nanoscale reactors, ion channels




Biomedical


-

Nanoelectrodes for implantation


-

Lab on a chip


-

DNA sequencing through AFM imaging


-

Artificial muscles


-

Vision chip for macular degeneration,



retinal cell transplantation

Electronics




CNT quantum wire interconnects




Diodes and transistors for


computing




Data Storage




Capacitors




Field emitters for instrumentation




Flat panel displays

Challenges

Challenges



Control of diameter, chirality



Doping, contacts



Novel architectures (not CMOS based!)



Development of inexpensive manufacturing


processes



Controlled growth



Functionalization with


probe molecules, robustness



Integration, signal processing



Fabrication techniques

SWNT

MWNT

Tower

Close view of MWNT Tower

MWNT Structures

Courtesy: Alan Cassell




Certain applications such as nanoelectrodes, biosensors would


ideally require individual, freestanding, vertical (as opposed to


towers or spaghetti
-
like) nanostructures




The high electric field within the sheath near the substrate in a plasma


reactor helps to grow such vertical


structures




dc, rf, microwave, inductive


plasmas (with a biased substrate)


have been used in PECVD of


such nanostructures

Cassell et al., Nanotechnology,
15

(1), 2004

Biosensor



High specificity



Direct, fast


response



High sensitivity



Single molecule


and cell signal


capture and


detection

3+

2+

e

3+

2+




Probe molecules for a given target can be attached to


CNT tips for biosensor development





Electrochemical approach: requires nanoelectrode




development using PECVD grown vertical nanotubes





The signal can be amplified with metal ion mediator


oxidation catalyzed by Guanine.

Courtesy: Jun Li




CNT tips are at the
scale

close
to

molecules




Dramatically
reduced
background noise

Traditional Macro
-

or

Micro
-

Electrode

Nanoelectrode

Array

Nanoscale electrodes create a dramatic improvement in signal
detection over traditional electrodes

Electrode



Scale difference

between macro
-
/micro
-

electrodes and molecules is
tremendous



Background noise

on electrode
surface is therefore significant



Significant amount

of target
molecules required



Multiple electrodes results in
magnified signal
and
desired
redundance
for statistical reliability.



Can be combined with other
electrocatalytic mechanism for
magnified signals.

Nano
-

Electrode

Insulator

Source: Jun Li

Functionalization of DNA

Cy3 image

Cy5 image

C. Nguyen et al, NanoLett., 2002, Vol. 2, p. 1079.

Electrochemical Detection

of DNA Hybridization

-

by AC Voltammetry

1
st
,
2
nd
, and
3
rd

scan in AC voltammetry

1
st



2
nd

scan: mainly DNA signal

2
nd



3
rd

scan: Background

1st

2
nd

and 3rd

#1
-
#2

#2
-
#3

Lower CNT Density


Lo睥raetectioLi浩t

J. Li, H.T. Ng, A. Cassell, W. Fan, H. Chen,
J. Koehne, J. Han, M. Meyyappan,
NanoLetters, 2003, Vol. 3, p. 597.

30 dies on a 4” Si wafer

200

m

300

m

Potential applications:

(1)
Lab
-
on
-
a
-
chip applications

(2)
Early cancer detection

(3)
Infectious disease detection

(4)
Environmental monitoring

(5)
Pathogen detection

Target

Molecule

1.
Chen, G.Y., Thundat, T. Wachter, E. A., Warmack, R. A., “Adsorption
-
induced surface stress and its
effects on resonance frequency of microcantilevers,”
J. Appl. Phys

77
, pp. 3618
-
3622 (1995).

2.
Ratierri, R.
et al
., “
Sensing of biological substances based on the bending of microfabricated
cantilevers,”

Sensors and Actuators B
61
, 213
-
217 (1999).


3.
Fritz, J.
et al.

“Translating Biomolecular Recognition into Nanomechanics,”
Science
288
, 316
-
318 (2000).


4.
Wu, G.
et al.

“Origin of nanomechanical cantilever motion generated from biomolecular interactions,”
PNAS
98
(4), 1560
-
1564 (2001).



Courtesy: Prof. A. Majumdar, U.C. Berkeley

Thiolated ssDNA

5’
-
HS

ATCCGCATTACGTCAATC

TAGGCGTAATGCAGTTAG
-
5’

(Complementary Strand)

Au

Self
-
Assembly of ssDNA

PB = Sodium Phosphate Buffer Solution

-

-

-

-

-

-

-

-

+

+

+

+

+

+

+

Wu, G.
et al.

“Origin of nanomechanical cantilever motion generated from biomolecular interactions,”


PNAS
98(4), 1560
-
1564 (2001).


Courtesy: Prof. A. Majumdar, U.C. Berkeley

Probe

ssDNA

Target ssDNA

Wu, G.
et al.

“Origin of nanomechanical cantilever motion generated from biomolecular interactions,”


PNAS
98(4), 1560
-
1564 (2001).


Courtesy: Prof. A. Majumdar, U.C. Berkeley

PSA

Wu, G.
et al
., “Bioassay of Prostate Specific
Antigen (PSA) Using Microcantilevers,”
Nature Biotechnology (Sept., 2001)

HSA
:

Human

Serum

Albumin

HP
:

Human

Plasminogen

fPSA
:

free

PSA

cPSA
:

complex

PSA

Courtesy: Prof. A. Majumdar, U.C. Berkeley

DNA Sequencing

Using Nanopores

Goal: Very rapid gene sequencing

(~2nm diameter)

-

Nanopore

in membrane

-

DNA in buffer

-

Voltage clamp

-

Measure current

G. Church, D.

Branton

, J.

Golovchenko

, Harvard

D.

Deamer

, UC Santa Cruz

a
-
hemolysin pore

Axial View

Side View

(very first, natural

pore)

Open nanopore

DNA translocation event



When there is no DNA translocation, there is a


background ionic current




When DNA goes through the pore, there is a drop in

the background signal




The goal is to correlate the extent and duration of the

drop in the signal to the individual

nucleotides

Source: Viktor Stolc

After a decade of using protein pores, efforts are
underway in many groups to develop synthetic pores
(such as in Si
3
N
4
)




Interaction with single nuclotides



-

~20 nucleotides in
a
HL simultaneously




Slower translocation



-

1
-
5

s /nucleotide in
a
HL




Resistance to extreme conditions



-

Temperature



-


pH



-

Voltage




a

-

hemolysin is toxic and hard to work with

Source: Viktor Stolc


Voltage
-
clamp amplifier designed to measure pA
level currents


Fast (up to 1GHz) data acquisition


Software for automatic blocking event detection
and recording

AgCl

AgCl

Voltage Clamp

Amplifier

nanopore

chip

KCl

KCl

Data Acquisition

G

G

A

A

A

A

G

C

C

T

T

Present

Future

A

A

G

G

G

G

C

C



Tree
-
like polymers, branching out from a central


core and subdividing into hierarchical branching


units



-

Not more that 15 nm in size, Mol. Wt very high



-

Very dense surface surrounding a relatively




hollow core (vs. the linear structure in traditional




polymers)




Dendrimers consist of series of chemical shells built on a


small core molecule



-

Surface may consist of acids or amines


means to attach functional groups







control/modify properties



-

Each shell is called a generation (G0, G1, G2….)



-

Branch density increases with each generation



-

Contains cavities and channels


can be used to trap guest molecules for




various applications.

Courtesy of:
http://www.uea.ac.uk/cap/wmcc/anc.htm



Desired features of effective drug delivery





-

Targeted delivery, controlled release (either timed or in response to an external





signal)




Desirable characteristics of dendrimers





-

Uniform size




-

Water Solubility





-

Modifiable surface functionality


-

Availability of internal cavity





-

Control of molecular weight



-

Control of the surface and internal structure




Number of different drugs can be encapsulated in dendrimers and injected into the body

for delivery





-

Incorporating sensors would allow release of drugs where needed




Gene Therapy





-

Current problem is getting enough genes into enough cells to make a difference.






Using viruses for this triggers immune reactions. Dendrimers provide an






alternative without triggering immune response




Cancer Therapy




Antimicrobial and Antiviral Agents



Dissolution kinetics may be the rate limiting step in the absorption process

for many drugs



-

Decreasing the particle size increases surface area and the dissolution



kinetics.




Liposomes are normally used as carrier for hydrophilic drugs. Typical

difficulties: physical instability, low activity, drug leakage



-

Alternative: water
-
soluble polymer based nanoparticles. These are



more site
-
specific and exhibit better controlled
-
release characteristics.



-

To overcome toxicity issues, solid lipid nanospheres as carrier systems



have been reported
*
. This is a lipid that is solidified and stabilized by



a surfactant.





Advantages: physical stability




Disadvantage: low drug loading (25%)

* S.A. Wissing et al., Adv. Drug. Del. Rev. 56, p. 1257 (2004).



Synthetic “droplets” containing anything from a single electron to

thousands of atoms but behave like a single huge atom.




Size: nanometers to microns




These are nanocrystals with extraordinary optical properties



-

The light emitted can be tuned to desired wavelength by




altering the particle size



-

QDs absorb light and quickly re
-
emit but in a different color



-

Colors from blue to IR




Common QDs: CdS, CdSe, PbS, PbSe, PbTd, CuCl…




Manufacturing



-

Wet chemistry



-

Template synthesis (zeolites, alumina template)

®

Energy levels

Absorption

Radiationless

decay

Fluorescence

Valence

Band

Band gap

Small

Molecules

Qdots

Semiconductors

Conduction

Band

Source: Bala Manian, Quantum Dot Corp.

®

2.2 nm


CdSe

5.0 nm


CdSe

E
g

~ 1/L
2

L

L

Source: Bala Manian, Quantum Dot Corp.





Quantum dots change color with size because additional

energy is required to “confine” the semiconductor

excitation to a smaller volume.



Ordinary light excites all color quantum dots.

(
Any
light source “bluer” than the dot of interest works.)

Source: Bala Manian, Quantum Dot Corp.

Material band
-
gap determines the emission range;
particle size tunes the emission within the range

Nanocrystal quantum yields are as high as 80%

Narrow, symmetric emission spectra minimize
overlap of adjacent colors

Source: Bala Manian, Quantum Dot Corp.



LEDs, solar cells, solid state lighting




Biomedical



-

Bioindicators



-

Lateral flow assays



-

DNA/gene identification, gene chips



-

Cancer diagnostics




Biological Labeling Agent



Broad output spectrum



Sharper spectrum



Fades quickly ~ 100 ps



5
-
40 ns



Unstable



Stable output over time



One dye excited at a time



Multicolor imaging, multiple





dyes excited simultaneously