Basics Basics

Nanomaterials

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Nano Materials Nano Materials

김기범

서울대학교 재료공학부

Nanomaterials

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Contents Contents

‰ Introduction

‰ Basics

‰ Synthesis of Nano Materials

‰ Fabrication of Nano Structure

‰ Nano Characterization

‰ Properties and Applications

Nanomaterials

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Basics Basics

‰ Crystal structure

‰ Surface

‰ Kinetics

‰ Surface chemistry

‰ Consolidation

‰ Quantum confinement

Nanomaterials

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Basics

Basics - - Crystal structure Crystal structure

‰ Crystalline vs. amorphous

Crystalline SiO₂ Amorphous SiO₂

Crystalline Si Matrix

Matrix ÆÆ Amorphous SiO Amorphous SiO₂₂

Long range order Short range order

Crystalline Au

10nm

Nanomaterials

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Basics

Basics - - Crystal structure Crystal structure

‰ Definition of Crystal

A crystal is an anisotropic, homogeneous body consisting of a three-dimensional periodic ordering of atoms, ions or molecules

‰ Definition of Lattice

A lattice means a three-dimensional array of points coinciding with atom positions (the space arrangement of equivalent sites in a crystal )

‰ Lattice Parameters

Nanomaterials

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Basics

Basics - - Crystal structure Crystal structure

‰ Crystal Structure

Crystal structure = Lattice + Basis

Nanomaterials

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Basics

Basics - - Crystal structure Crystal structure

‰ Definition of Crystallographic direction

Crystallographic direction is defined as a line between two points, or a vector.

Note that the direction of [uvw] describes not only a line through the origin and the point uvw, but the infinite set of lines which are parallel to it.

Nanomaterials

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Basics

Basics - - Crystal structure Crystal structure

‰ Definition of Crystallographic plane

The values ( h k l ) are called Miller indices, and they are defined as the smallest integral multiples of the reciprocals of the plane intercepts of axis.

Nanomaterials

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Basics

Basics - - Crystal structure Crystal structure

‰ Seven Crystal Systems

Nanomaterials

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Basics

Basics - - Crystal structure Crystal structure

‰ 14 Bravais Lattice

Nanomaterials

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Basics

Basics - - Crystal structure Crystal structure

‰ Close Packed Structure- FCC

Nanomaterials

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Basics

Basics - - Crystal structure Crystal structure

‰ Close Packed Structure- HCP

Nanomaterials

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Basics

Basics - - Crystal structure Crystal structure

‰ Diamond

Si, Ge 1 1 1

4 4 4

8 atoms/unit cell

coordination #: 4:4

FCC with two atoms per lattice point, (0,0,0) and ( , , )

two interwoven FCC lattices

1

2

tet. sites

by cations

Nanomaterials

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Basics

Basics - - Crystal structure Crystal structure

‰ Zinc Blende

GaAs

1

2

tet. sites by cations

1 1 1 4 4 4

4 molecules/unit cell coordination #: 4:4

FCC with two different atoms per lattice point, (0,0,0) ( , , )

two interwoven FCC lattices

Nanomaterials

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Basics

Basics - - Crystal structure Crystal structure

‰ Wutzite

Nanomaterials

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Basics

Basics - - Crystal structure Crystal structure

‰ Carbon - diamond - graphite

ƒwww.spmtips.com/products/hopg/

o

covalent/van der Waals 1.48/3.4 A

4 atoms/unit cell coordination #: 3

Basics

Basics - - Crystal structure Crystal structure

‰ Carbon - graphene

- fullerenes (C₆₀) - nanotube

A.K.Geim, Nature Mater., 6, 183 (2007)

‰ Carbon-Classification of carbon nanotubes

- Single-wall CNT, double-wall CNT, multi-wall CNTs - Zigzag and armchair nanotubes, achiral nanotube

SWCNT DWCNT Thin-MWCNT MWNCT

Basics

Basics - - Crystal structure Crystal structure

‰ Carbon-Classification of carbon nanotubes

- Zigzag and armchair nanotubes, achiral nanotube

Basics

Basics - - Crystal structure Crystal structure

Basics

Basics - - Defects Defects

‰ Point Defect

- Vacancy - Interstitial

- Impurities (Dopants)

P. Ebert, Materials Today, 6, 36 (2003)

Basics

Basics - - Defects Defects

'

oxygen vacancy: V +2e

Basics

Basics - - Defects Defects

Basics

Basics - - Defects Defects

‰ Line Defect (dislocation)

- edge - screw

Burgers vector, b a)

Dislocation line

Nanomaterials

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Edge Screw Mixed

Basics

Basics - - Defects Defects

‰ Line Defect (dislocation) - mixed

Nanomaterials

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Basics

Basics - - Defects Defects

‰ Line Defect (dislocation) - observation of dislocation

high resolution transmission electron microscope image electron beam incident along an <011> zone of Si

12

b= r [011]

Nanomaterials

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Basics

Basics - - Defects Defects

‰ Line Defect (dislocation)

- lattice fringe image of dislocation

narrow edge dislocation wide edge dislocation

Nanomaterials

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transmitted beam-bright field image-dislocation-dark

diffracted beam-dark field image -dislocation-bright

Basics

Basics - - Defects Defects

‰ Line Defect (dislocation) - diffraction contrast

Nanomaterials

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Basics

Basics - - Surface Surface

‰ Surface Energy

‰ Estimation of surface energy

‰ Anisotropy FCC

-2 , ,

-

: surface energy [Jm ]

i

i i

i

P T n

dG SdT VdP dn dA

G A

μ γ

γ

= + + +

⎛ ∂ ⎞

= ⎜⎝ ∂ ⎟⎠

∑

1

2 N_{b a} N_b : # of broken bonds, a : surface atomic density

γ = ερ ρ

} 2 111

{ 2 3

a

γ

ε

2 5

a

γ

ε

2 } 2

100 {

4 4 2 2 1

a a

ε ε

γ = =

a

Nanomaterials

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Basics

Basics - - Surface Surface

‰ Anisotropy of Surface Energy

4x1x250=1000 erg

(4x0.32x250)+(4x0.59x225)

=851 erg 1 cm² 1 cm²

1 cm²

4x1x225=900 erg

A.W. Adamson, Physical Chemistry of Surfaces (1982)

‰ Equilibrium shape - total surface energy

minimization

Basics

Basics - - Surface Surface

Negative crystal UO₂

Y.-M Chiang, Physical Ceramics (1997) Nanomaterials

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NaCl Ag

Ag Au

i Chi

γ =

‰ Wulff plot

- γ-plot- curved outer line

- crystal shape- inner envelope

G. CaO, Nanostructures & Nanomaterials (2004)

Nanomaterials

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Basics

Basics - - Surface Surface

‰ Surface Energy ‰ Surface Energy vs. size

Y.-M Chiang, Physical Ceramics (1997) G. CaO, Nanostructures & Nanomaterials (2004)

Nanomaterials

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Basics

Basics - - Surface Surface

‰ Curved Surface

2

1 2

1 2

4 8 for sphere

2

1 1

Laplace equation ( )

, : principal radii of curvature PdV dA

dV r dr dA rdr P dA

dV r

P r r

r r

γ

π π

γ γ

γ

Δ =

= =

Δ = =

Δ = +

Nanomaterials

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Basics

Basics - - Surface Surface

‰ Curved Surface

3

ln ln ln

( ) 4

ln 8 ,

( ) 3

2 1

( ) ( ) exp[ ( )]

: Kelvin equation

o o o

m

m

RT a RT c RT P

RT P r dA rdr V r

P r P r P r V

RT r

μ μ μ μ

μ γ γ π π

γ

= + = + = +

Δ = = = =

= ∞

= = ∞

- pressure difference across a curved surface → a change in solubility or vapor pressure

- at constant T, P, n_i, transfer one mole from plat to curved

Nanomaterials

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Basics

Basics - - Surface Surface

‰ Solubility

‰ Solubility ‰ Vapor Pressure

G. CaO, Nanostructures & Nanomaterials (2004)

Nanomaterials

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Basics

Basics - - Surface Surface

‰ Melting Point

- melting point ↓⇔ particle size ↓

Ph. Buffat, Phys. Rew., A13 (1976) 2287

2 / 3

2 _{b s}

b m s l

s s l

T T T

H r

γ γ ρ

ρ ρ

⎡ ⎤

⎡ ⎤ ⎛ ⎞

⎢ ⎥

− = ⎢⎣Δ ⎥⎦⎢⎣ − ⎜ ⎟⎝ ⎠ ⎥⎦

Au

K. Ishikawa, Phys. Rew., B37 (1976) 5852

‰ Transition Temperature

PbTiO₃

Nanomaterials

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Basics

Basics - - Surface Surface

‰ Melting Point of nano-wire - Ge nano-wire

Y. Wu, Adv. Mater., 13 (2001) 520 D. Quere, Science, 249 (1990)1256.

- Rayleigh instability

Nanomaterials

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Basics

Basics - - Surface Surface

‰ Melting Point & Lattice Parameter - CdS nanocrystal

A.N.Goldenstein, Science, 256 (1992)1425.

Nanomaterials

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Basics

Basics - - Surface Surface

‰ Ostwald Ripening

J. B. Hannon, Phys. Rev. Lett., 79 (1997) 2506

2 1

(1

)

r o

C C V

RT r

= + σ

Nanomaterials

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Basics

Basics - - Surface Surface

‰ Capillary Rise

2 2 cos

2 cos

0 90 90< <180

c

P gh R gh

r R

θ ρ

γ γ ρ γ

θ

θ θ

⎛ ⎞

Δ = ⎛ ⎞ ⎜ ⎟ ⎝ ⎠ = ⎜ ⎝ ⎟ ⎠ = =

< <

θ

Nanomaterials

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Basics

Basics - - Surface Surface

‰ Wetting & Spreading - contact angle

- wetting

γ_SV=γ_SL+γ_LV cos θ θ: contact angle Young equation

nonwetting θ > 90 wetting θ < 90 spreading θ = 90

W. D. Kingery, Introduction to Ceramics (1976)

Nanomaterials

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Basics

Basics - - Surface Surface

‰ Wetting

- grain boundary

W. D. Kingery, Introduction to Ceramics (1976)

φ: dihedral angle

Nanomaterials

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Basics

Basics - - Kinetics Kinetics

‰ Homogeneous Nucleation - supersaturation (ΔGv)

- surface

4 r π γ

4 3

3

π r Δ G

*

4 3 2

3

*

*

*

3

*

4 critical radius

( ) 2

0 r

barrier height 16

3

V

r r V

V

G r G r

r d G

dr G

G G r

G

π π γ

γ

π

=

Δ = Δ +

Δ −

⎛ ⎞ = → =

⎜ ⎟ Δ

⎝ ⎠

Δ

Δ =

Δ

W. D. Callister, Materials Science and Engineering spherical nuclei

Basics

Basics - - Kinetics Kinetics

‰ Homogeneous Nucleation ex) supersaturated solution

43 Nanomaterials

o o

o v

C C

C

C kT kT C

G

/ ) (

) 1

ln(

) /

ln(

−

=

Ω +

− Ω =

−

= Δ

σ

σ

ƒ For nanoparticles

- T

-

Gv σ

γ

Δ ⇑ ← ⇑ ← ⇓

⇓

E 1 2 3

T >T >T >T

Nanomaterials

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Basics

Basics - - Kinetics Kinetics

‰ Nucleation Rate

. *

*

1 2

~ (# of stable nuclei) (collision frequency)

~ exp( ) exp( )

d d

N n v

Q

K G K

kT kT

×

− Δ −

W. D. Callister, Materials Science and Engineering

Nanomaterials

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Basics

Basics - - Kinetics Kinetics

‰ Mono-dispersed Particle - Lamar diagram

T. Sugimoto, Adv. Colloid Interface Sci., 28 (1987) 65 How to achieve the uniformity in size?

1. High rate of nucleation

2. Quick down to the minimum concentration.

Æ prevent further nucleation

Nanomaterials

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Basics

Basics - - Kinetics Kinetics

‰ Homogeneous Nucleation - subsequent growth

(1) diffusion controlled (2) interface controlled

2 2 2

2 2

) (

) (

) (

2

) (

o D

o o

o o

o D

o m

S m S

r t k

r r

r r r r

r t k r

r t V C

C D r

r C V

C dt D

dr

= +

=

+

=

+

−

=

−

=

δ δ δ

2

2

2

(2-1) monolayer growth

1 1

m ; m

o

o o

dr k r k t

dt r r

r r r

r δ δ

= = −

⎛ ⎞

= ⎜ ⎟

⎝ ⎠

(2-2) multilayer growth

p; p o

o

dr k r k t r dt

r r

δ δ

= = +

=

Nanomaterials

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r δr

(or )

r t

(2-1)

(2-2) (1)

Basics

Basics - - Kinetics Kinetics

‰ Homogeneous Nucleation - for the uniformity in size

Æ diffusion controlled process is desired

- how to achieve it

Æ extremely low concentration of growth species

high viscosity, diffusion barrier, controlled supply of growth

species

Nanomaterials

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Basics

Basics - - Kinetics Kinetics

‰ Homogeneous Nucleation ex) ZnS

R.Williams et al., J. Colloid Interface Sci. 106, 388 (1985).

ƒ diffusion of the HS- ion to the growing particle is the rate-limiting process

Nanomaterials

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Basics

Basics - - Kinetics Kinetics

‰ Heterogeneous Nucleation * * ²

homo 3

* *

homo

3

sin cos 2 cos 2 2 3cos cos

( ) 2 3cos cos

( ) 4

heter

heter

r r

G G f

f

θ θ θ

θ θ

θ

θ θ

θ

+ −

= − +

Δ = Δ

− +

=

W. D. Callister, Materials Science and Engineering

Nanomaterials

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Basics

Basics - - Surface chemistry Surface chemistry

‰ Dispersion

- a high solids homogeneous suspension with a well-defined rheological behavior

‰ Dispersion in liquid

- wetting - interparticle interaction

R. J. Pugh, Surface and Colloid Chemistry in Advanced Ceramic Processing

Nanomaterials

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Basics

Basics - - Surface chemistry Surface chemistry

‰ Stabilization

R. J. Pugh, Surface and Colloid Chemistry in Advanced Ceramic Processing

Nanomaterials

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Basics

Basics - - Surface chemistry Surface chemistry

‰ Electrostatic Stabilization electrical repulsion

van der waals attraction

‰ Electrical double layer - surface charge (Ψ_o)

- stern potential (Ψ_d)

specific adsorption of counter-ion - electrockinetic or ζ(zeta) potential

at plane of shear (slipping plane) - double layer thickness (κ^-1)

D.J.Shaw, Introduction to colloid and surface chemistry, 1992

M_i : molar concentration

Nanomaterials

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Basics

Basics - - Surface chemistry Surface chemistry

‰ Van der Waals attraction

induced dipole- induced dipole (London) ~ x^-6 permanent dipole- induced dipole (Debye) ~ x^-6

permanent dipole- permanent dipole (Keesom) ~ x^-6

‰ Attraction between two spheres A: Hamaker constant

A 12

Ar S Φ = −

G. CaO, Nanostructures & Nanomaterials (2004)

Nanomaterials

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Basics

Basics - - Surface chemistry Surface chemistry

‰ Example- CdTe nanowire

dipole-dipole interaction between nano-particles produce nano- wires with self assembling modes

Z. Tang, Science, 297 (2002)237

intermediate stage nano-wire

Nanomaterials

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Basics

Basics - - Surface chemistry Surface chemistry

‰ DLVO theory

http://www.zeta-meter.com/

primary minima

secondary minima kinetic barrier

P. C. Hiemenz, Principles of colloid and surface chemistry (1986)

R A

Φ = Φ + Φ

10^-2 M 10^-3

M

Basics

Basics - - Surface chemistry Surface chemistry

‰ DLVO theory - effect of

Hamaker constant surface potential electrolyte conc.

P. C. Hiemenz, Principles of colloid and surface chemistry (1986)

Basics

Basics - - Surface chemistry Surface chemistry

‰ Steric Stabilization

D. H. Napper, Polymeric Stabilization of Colloidal Dispersions (1983)

Steric stabilizer : amphipathic block or graft copolymer

‰ Metal oxide nanoparticles in aqueous suspensions

- kinetic stability- energy barrier (DLVO theory)

- dispersion, aggregation, flocculation - thermodyamic stability- surface energy minimization

- Ostwald ripening (dissolution-reprecipitation)

* Is it possible to avoid the ripening of nanoparticles in suspension and to control their dimension by monitoring the precipitation conditions?

ex) thermodynamically stable dispersed system- microemulsion answer) possible

When the pH of precipitation is sufficiently far from the point of zero charge and the ionic strength sufficiently high, the ripening of nanoparticles is

avoided. The stability condition, defined by a 'zero' interfacial tension, corresponds to the chemical and electrostatic saturation of the water-oxide interface. In such a condition, the density of charged surface groups reaches its maximum, the interfacial tension its minimum and further adsorption forces the surface area to expand and consequently, the size of nanoparticles to decrease.

Basics

Basics - - Stability of Stability of Nanoparticle Nanoparticle

L. Vayssieres, Int. J. Nanotech. 2, 411 (2005)

‰ Microemulsion

- clear, stable, isotropic liquid mixtures of oil, water, and surfactant, frequently in combination with a co-surfactant.

- aqueous phase may contain salt(s) and/or other ingredients, and the “oil”

may actually be a complex mixture of different hydrocarbons and olefins.

- thermodynamically stable

- interfacial tension is very low (10^-2~10^-3 mN/m)

B.K. Paul, Current Science 80, 990 (2001)

Basics

Basics - - Stability of Stability of Nanoparticle Nanoparticle

E. Ruckenstein, Chem. Phys. Lett. 57, 517 (1978)

‰ Metal oxide nanoparticles in aqueous suspensions

Basics

Basics - - Stability of Stability of Nanoparticle Nanoparticle

L. Vayssieres, Int. J. Nanotech. 2, 411 (2005) S.M.Ahmed, J. Phys. Chem. 73, 3546 (1969)

point of zero interfacial tension

stable

- no growth unstable

-ripening

γ decreases as pH increases

Nanomaterials

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Basics

Basics - - Consolidation Consolidation

‰ Consolidation (sintering)

processes involved in the heat treatment of powder compacts at elevated temperatures, usually at T > 0.5Tm [K], in the temperature range where diffusional mass transport is appreciable resulting in a dense polycrystalline solid.

- pore removal densification

J. P. Schaffer et al, The Science and Design of Engineering Materials

MgO-doped Al₂O₃

Nanomaterials

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Basics

Basics - - Consolidation Consolidation

‰ Solid state sintering

initial intermediate final

Nanomaterials

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Basics

Basics - - Consolidation Consolidation

‰ Densification vs. Grain Growth

500μm

TiO₂ & SiO₂-doped Al₂O₃

O. S. Kwon, Acta Mater., 50 (2002) 4865-4872