Science and Engineering of Organic Nanomaterials

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Carbon NanomaterialsNanomaterials Design Laboratory (CNDL)Design Laboratory (CNDL)

Science and Engineering of Science and Engineering of

Organic

Organic Nanomaterials Nanomaterials

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Part I. Fundamentals:

Basics For Designing Basics For Designing Organic

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Part I. Fundamentals:

Part I. Fundamentals:

Basics For Designing Organic

Basics For Designing Organic NanomaterialsNanomaterials

Chapter 1. Physical Properties of Chapter 1. Physical Properties of

Organic

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What Is Nanoscale ?

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Definition

the systematic manipulation of matter on the length scale 1-100 nm to produce useful new engineered structures, materials, or devices.

History

- R. Feynman, 1959, “There’s plenty of room at the bottom”

- Kroto, Smalley, Curl, 1985, synthesis of Fullerene, C60

- Sumio Iijima, 1991, discovery of single wall carbon nanotube

Nanotechnology today

- 1600 companies, 700 industrial products - world-wide investment of 4.8 billion $ / yr

The U.S. National Nanotechnology Initiative (NNI) - first passed in 2001

- now ~ 1 billion $ / yr

- optimistic status report: May 2005

- four phases envisioned (now in phase 1)

The Nanotechnology Movement

about 1/1000 width of human hair

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Nanotechnology is the development of….

Nanoparticles

- buckminster fullerene C₆₀

- metallic nanoparticles (gold, silver, iron, etc) - glass/ceramic nanoparticles (titania, silica, etc) - quantum dots (semiconductor nanoparticles)

Nanotubes, nanofibers, nanowires

- carbon nanotubes

- nanofibers (carbon, polymer, ceramic) - nanowires (metallic, semiconducting)

Nanoplatelets

- graphite, clay,

- silica, hydroxyapatite

Nanostructured surfaces Nanostructured solids

and their application as

components for next-generation technologies, including:

batteries, fuel cells, sunscreens, cosmetics, structural materials, implants, drug delivery vehicles, catalysts, sorbents, and

much, much more…….

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0-D materials 1-D materials 2-D materials

metals

semiconductors

ceramics

carbons

polymers

other

The Nanomaterial Matrix

(some well-known examples)

Au nanoparticles Fe magnetic nanoparticles

nanopart. catalysts (Pt,Zn,Cu,Ni,Co…) quantum dots

nanophase powders for low-T sinterable coatings, parts, composite fillers, sorbents

oligomers (e.g. chitosan) branched compounds (dendrimers)

surfactant assemblies (liposomes)

nano-clays

fullerenes, carbon black, nanohorns carbon nanotubes,

nanofibers nano-graphite semiconducting

nanowires, nanorods nanowires

electrospun ceramic nanofibers for composite fillers

electrospun polymer nanofibers biodegradable polymer nanobeads

for medical applications

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General Properties of

General Properties of Nanoscale Nanoscale System System

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Bulk Vs. Surface

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Bulk Vs. Surface

nano 10³

10^-9 ~ 10^-8

meso 10⁷

10^-8 ~ 10^-6

Cluster molecule

<10²

<10^-9

macro 10¹⁰

10^-6 ~ 10^-5

macro

>10¹⁴

> 10^-5

Feature Surface

atoms (%)

# of Size (nm) atom

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Surface Ratio

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Electrical Property

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Transport Properties of Nano-scale Materials

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Transport Properties of Nano-scale Materials

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Feature of Nanoscale System

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In Nanoscale System

Newton mechanics – particle deterministic description Blackbody radiation

Photoelectric effect Compton effect Atomic spectra

Introduction of quantized concept

Unsolved problem

Quantum size effect

As the size of system decrease, new effects begin to appear - # of charge carrier

n type semiconductor, n_d = 10¹⁸/cm³

but in nanosystem, n_d = 1/(10nm)³ only one electron - density n states 1,2,3 dimension

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In Nanoscale System

(Quantum size effect)

Density of state (DOS)

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10 nm

Fe₃O₄, Magnetite

Size-dependent Magnetism

Size-Dependent Emission

CdSe Semiconductor

50 nm

Nanogold on Silica

Size-dependent Absorption

HALAS, WEST

In Nanoscale System

(Quantum size effect)

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Two Approach in Nanotechnologyn

• Fabrication of nanomaterials/nanodevices by lithography

• Ultrahigh density device fabrications

• Complicated and of high cost

• Use of limited materials due to material compatibility

• Fabrication of nanodevices by simple assembly of nanomaterials

• Numerous types of nanodevices

• Use of numerous nanomaterials

(Carbon nanotubes, semiconductor nanocrystals and nanowires/nanorods, etc)

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Nanotechnology & Nanomaterials

Top-down devices

Bottoms-up materials

Ultra-fine particles Carbon black

Colloid Silica Historical

Carbons Inorganic materials Polymers Composition

Nanocomposites Nanotubes Nanocrystals

Format

Nanopores

Classification

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…

Nano lithography / Patterning

Bulk Materials Nano-scale structures

• E-beam lithography

• Nanoimprint Technology KrF

(λ = 248 nm)

ArF (λ = 193 nm)

F₂

(λ = 157 nm)

EUV (λ = 11.4 nm)

Conventional Lithography

Nanolithography

• Probe Technology

Demand

High controllability High cost

Top-down Approach

R. Feynman, 1959,

“There’s plenty of room at the bottom”

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Carbon NanomaterialsNanomaterials Design Laboratory (CNDL)Design Laboratory (CNDL) Nano-scale materials

Nucleation / Growth

Atoms / Molecules Nano-scale structures

Cost effective

Low controllability

Self-assembly

• Colloid process

• Thin film technology

• Gas phase nucleation

Size Distribution Areal Density Distribution

Nucleation, Growth & Self-assembly

Bottom-up Approach

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Introduction to Solid State Physics

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long range Vs. short range order

crystal glass gas

crystals: constituents fall into regular, repeated 3D patterns long range order and short range order

anisotropic geometry anisotropic properties other solids: amorphous

examples: glass, some plastics, etc.

statistically isotropic on a long range anisotropic on a short range

possibly short range order

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Line lattice

Plane lattice

a

a b γ

Lattice Structures

Crystal axis Unit cell

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Crystal structure = Lattice + Basis

Basis

a

b

A atom

C atom B atom

+

Lattice point

Triclinic Monoclinic

Orthorhombic

Tetragonal Hexagonal

Cubic Trigonal

7-crystal system

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14 Bravais Lattices

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Closed Packed Structures

Layer Stacking sequence Layer Stacking sequence

2D Hexagonal Packing

3D ABABABABAB (HCP)

3D ABCABCABC (FCC)

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Interstitial Sites ; FCC structure

Octahedral sites ; 4

Octahedral sites ; 4 Tetrahedral sites ; 8Tetrahedral sites ; 8

The number of octahedral sites is equal to the number of spheres The number of tetrahedral sites is double of the number of spheres

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Carbon NanomaterialsNanomaterials Design Laboratory (CNDL)Design Laboratory (CNDL) 8 )

, 5 0 , 0 (

Tetrahedral sites ; 4 Tetrahedral sites ; 4

8 ) , 1 3 , 2 3 (1 8)

, 3 0 , 0

( )

8 , 7 3 , 2 3 (1

Interstitial Sites ; HCP structure

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In FCC, the relation between layers and atom number

The number of atoms on the surface is

Structural magic number

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Lattice Vibrations

In reality, a crystal undergoes continuous fluctuations in the neighborhood of their regular positions in the lattice. These fluctuations arise from the heat or thermal energy in the lattice, and become more pronounced at higher temperatures.

Normal modes : Lattice’s own characteristic modes or frequencies of vibration, and the overall collective vibrational motion of the lattice is a combination or superposition of many normal modes

Acoustic modes : low-frequency mode

Optical modes : high-frequency mode

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Diatomic vibration

(acoustical and optical branches)

(ref) http://www.chembio.uoguelph.ca/educmat/chm729/Phonons/optmovie.htm

Transverse acoustical mode for diatomic vibration

Transverse optical mode for diatomic vibration

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1-D Lattice with Diatomic Atom

Consider a linear diatomic chain of atoms (1-D model for a crystal like NaCl):

In equilibrium:

M₁

a

M₂ M₁ M₂

Applying Newton’s second law and the nearest-neighbor approximation to this system gives a dispersion relation with two “branches”:

2 / 1

2 2 1 2

1 2 1 2

2 1

2 2 1

1 2

1

2 1

1

2

4 sin ( )

⎥ ⎥

⎦

⎤

⎢ ⎢

⎣

⎡ ⎟⎟ ⎠ −

⎜⎜ ⎞

⎝

⎛ +

⎟⎟ ±

⎠

⎜⎜ ⎞

⎝

⎛ +

= ka

M M

c M

M

M c M

M M

M c M

ω

ω_-

(k) ω

Æ

0 as k (=2π/λ) Æ 0 acoustic modes (M

₁

and M

₂

move in phase)

ω₊

(k) ω

Æ ω_max

as k Æ 0 optical modes (M

₁

and M

₂

move out of phase)

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These two branches may be sketched

schematically as follows:

gap in allowed frequencies

k

ω

a π a

− π

0

optical

acoustic

In a real 3-D solid the dispersion relation will differ along different directions in k-space. In general, for a p atom basis, there are 3 acoustic modes and (p-1) groups of 3 optical modes, although for many propagation directions the two transverse modes (T) are degenerate.

1-D Lattice with Diatomic Atom

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Phonon

hω Energy

Distance

Equilibrium distribution

1 exp

1

⎟⎟−

⎠

⎜⎜ ⎞

⎝

= ⎛

T k n

B

hω

• where ħ_w can be thought as the energy of a particle called phonon, as an analogue to photon

• n can be thought as the total number of phonons with a frequency w, and follows the Bose-Einstein statistics:

h ω

⎟ ⎠

⎜ ⎞

⎝ ⎛ +

= 2

n 1 u

•The linear atom chain can only have N discrete K Æ w is also discrete

• The energy of a lattice vibration mode at frequency w was found to be

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conductivity of solids varies strongly:

•Cu r=1.7x10

^-8

W (good conductor)

•quartz r=7.5x10

¹⁷

W (good insulator)

•explanation: electron bands!

internuclear distance

energy

example: 2 Na atoms

Na+Na Na

₂

molecular IP

atomic IP

1 attractive

and 1 repulsive state

Band Theory of Solids

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Carbon NanomaterialsNanomaterials Design Laboratory (CNDL)Design Laboratory (CNDL) internuclear distance

energy

attractive

repulsive

overlapping wave functions lead to splitting of the state”:

•attractive state: more electron density between atoms, more screening

•repulsive state: less electron density between atoms, less screening

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as many states as atoms involved!

5 Na atoms:

energy

“continuous”energy band of closely spaced levels 10

²³

Na atoms:

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electron bands and forbidden bands

“atomic” state

overlapping bands empty band

filled bands

conductor

“atomic” state filled band

partially filled band

conductor

Î

_F

Î

_F

“atomic” state forbidden band

“gap”

empty band

filled bands

Î

_F

semiconductor or insulator

“small” gap (~1 eV):

semiconductor

“large” gap (several eV):

insulator

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conductors:

•valence band is either only partially filled or overlaps with an empty band

•an electric field accelerates electrons

•with their increased energy they still “fit” into the empty part of the band

•conductors are opaque insulators:

•valence band is filled, the gap to the empty band is several eV

•electrons cannot overcome this gap thermally

•the electric field has to reach 10

⁸

V/m to overcome the gap (smaller fields are insufficient because of collisions)

•the material is called insulator

•insulators are usually transparent

electron bands and forbidden bands

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T~0 K:

•semiconductors and insulators have almost identical resistivity room temperature

•in semiconductors a tiny fraction of electrons can be thermally excited over the gap to the empty band

•the few electrons are sufficient to allow small current flows in the presence of a field E

“gap”

almost empty band, conduction band

almost filled band, valence band

Î

_F

Semiconductors

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Î

_F

n-type:

• replace few Si atoms by e.g. As

• Si has 4 valence electrons needed for covalent bond

• As has 5 valence electrons 1 excess electron

• excess electron needs fractions of eV to reach the conduction band

• excess electron state is called donor level

• Fermi energy is raised towards the conduction band

p-type:

• same principle, but one electron too little

• e.g. replacement of Si by Ga

• excess vacancy, excess hole

• electron from the valence band can easily reach the so called acceptor levels

donor levels

Î

_F

acceptor levels

Doping

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Exciton

An exciton is a bound state of an electron and an imaginary particle called an electron hole in an insulator or semiconductor, and such is a Coulomb-correlated electron-hole pair. It is an elementary

excitation, or a quasiparticle of a solid.

1. photon enters a semiconductor

2. exciting an electron from the valence band into the conduction band.

3. The missing electron in the valence band leaves a hole behind, of opposite electric charge, to which it is attracted by the Coulomb force.

4. The exciton results from the binding of the electron with its hole;

as a result, the exciton has slightly less energy than the unbound electron and hole.

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Excitons

(bound electron-hole pairs)

SEMICONDUCTOR PICTURE

GROUND STATE WANNIER EXCITON

binding energy ~10meV radius ~100Å

( )

_⎭^⎬^⎫

⎩⎨

⎧

= +

=

h e

e ex

m m m m

n eV m E m

/ 1

/ / 6 . 13

* 2 2 0

0

*

ε ε

fig. 2.20

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Various Organic

Various Organic Nanomaterials Nanomaterials : Its Physical Properties

: Its Physical Properties

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Part Ⅰ . Fundamentals:

Basics for Designing Organic Nanomaterials

Chapter 1. Physical Properties of Organic Nanomaterials

Chapter 2. Design of Nanomaterials based on Microemulsion Physics Chapter 3. Design of Nanomaterials based on Sol-gel Chemistry

Chapter 4. Design of Nanomaterials based on Vapor Processing

Chapter 5. Design of Nanomaterials based on Macromers and Block

Copolymers

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Chapter 1. Physical Properties of Organic Nanomaterials

1.1. Introduction to nanomaterials

1.2. Size-dependant Properties 1.2.1. Melting points

1.2.2. Magnetism

1.2.3. Optical properties - Color 1.2.4. Conductivity

1.3. Applications 1.3.1. Colors 1.3.2. Catalysis

1.3.3. Nanoelectronics

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1.2. Size-dependant Properties

To demonstrate quantum size effects experimentally, appropriate materials are needed.

In the course of the last 10-15 years there has been remarkable progress in

preparing metal clusters in the size range of less than one nanometer up to dozens of nanometers.

Here, only some selected examples are given that have turned out to exhibit well- established size-dependent properties

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1.2.1. Melting points

1.2. Size-dependant Properties

 Assumption

: A property is not directly dependent on the electronic situation in the particles, but is rather a consequence of the averaged coordination number of the participating atoms.

: The majority of metals form hexagonal or cubic close-packed structures with coordination numbers of 12.

: The coordination numbers of the surface atoms are 9 or smaller, depending on faces and kind of faces, edges, or corners.

 Bulk vs. Nanoparticles

- Bulk materials  the surface atoms form a negligible part of the total number of atoms

- The smaller particles  the proportion of surface atoms increases

 The information on the percentage of surface atoms in close-packed full-shell clusters of different sizes.  Table 1.1

(Ex. A spherical particle of 50nm in diameter  only about 6% of surface atoms)

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1.2.1. Melting points

Table 1.1 The relation between the total number of atoms in full shell clusters and the percentage of surface atoms

- Full-shell clusters: built up by hexagonal (HCP) or cubic (CCP) closed-packed atoms

- The case in most bulk metals consisting of a central atom and surrounded dense-packed shells.

- The number of atoms per shell

: ₂

10n 2 ( = number of shells)n

  

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1.2.1. Melting points

 The melting point of solid is reached when the order of the lattice is beginning to be destroyed.

 Bulk vs. Nanoparticles

- Bulk : For a distinct solid, the melting point is a physical constant.

- Nanoparticles : When the number of surface atoms becomes equal to or even exceeds the number of inner-core atoms, the melting process starts earlier.

 As the coordination number of surface atoms is 9 or smaller, these atoms are more easily rearranged than those in the center of the particles.

 In Figure 1.1, the relation between particle size and melting point of gold particles is shown, calculated by the method of Reifenberger.  Ref.2-1

: dramatic decrease of melting points for particles smaller than 3-4 nm

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1.2.1. Melting points

Figure 1.1 Relation between the size of gold particles and their melting point

 We can see that even thermodynamic properties of matter, which classically described are natural constants, change with the dimension of the material.

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1.2.2. Magnetism

 Diamagnetic materials have only spin-paired electrons.

 In practice a macroscopic piece of a diamagnetic metal can have one or unpaired electrons. No way to measure because of the effectively infinite number of atoms and electrons.

 However, the particle size  small enough to make one unpaired electron measurable  the predicted so-called “odd-even effect” become visible

※ Odd-even effect : Among small diamagnetic metal particles, there should be a 50:50 distribution of odd and even numbers of electrons

 Details in Chapter 2

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1.2.2. Magnetism

►Example◄

Y.VOLOKITIN, J.SINZIG, L.J.DE JONGH, G.SCHMID, M.N.VARGAFTIK and I.I.MOISEEVI,

Nature, 1996, 384, 621

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 Metals can be polarized by electromagnetic irradiation exciting electrons.

 Considering the ideal case of an undamped oscillation of free electrons in a defect-free metal lattice, the solution of the wave eq. results in an expression for the complex refractive index n:

 Appendix 1-1

 The refractive index is related to the reflectivity R_v of a metal by the Beer equation.

1.2.3. Optical properties - Color

2 2

2 0

1 ^E

e

n e N

 m v

 

2 2

( ' 1) ( ' 1)

v

n k

R

n k

 

  

N_E : the density of electrons m_e : electronic mass

v : frequency of the electromagnetic irradiation ε₀ : dielectric constant of vacuum

k : absorption coefficient

n’ : real part of the complex refractive index n = n’(real part) + n’’(imaginary part)

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1.2.3. Optical properties - Color

2

0 E P

e

v e N

 m



 If v is small enough  n² <0 (n consists of only the imaginary part n’’  n’=0

 the refractivity R=1 : reflect electromagnetic irradiation totally

 Higher frequencies  n’>1 with k=0 (nonabsorbing metals)

 n≈1 and R≈0 : ideal metals under these conditions become transparent (for instance, alkali metals in the near UV-region)

 The limiting or plasma frequency v_p for the transition R=0  R=1 for free electron

 If metal particle size is reduced  the mean free path of the electrons exceeds the particle’s radius  electrons can be excited by visible light to perform fluidlike plasmon oscillations : Mie theory

 The dipole and higher multiple moments are caused by surface charging (especially effective for spherical shapes)

light

electric field

surface charges

++ + + +

- -

- - -

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1.2.3. Optical properties - Color

 As can be seen from Figure 1.3 , the varying intensity of the plasmon resonances depends on the cluster size.

 Decreasing particles size  increasing damping  the electronic relaxation after electromagnetic excitation is accelerated  reduced life time of the plasma excitation  broadening of the lines.

Figure 1.3 Absorbance spectra of (a) silver and (b) gold clusters of different sizes

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1.2.3. Optical properties - Color

 The above-mentioned optical properties refer to isolated nanoparticles. In practice, the particles often cannot be considered as independent from each other.

 Aggregates of clusters effect shifts of the plasmon resonances depending on their topologies.

 The orientation of the dipoles of connecting particles results in two individual vibrations of the optical excitation.  Fig 1.4

 Excitation along the row causes a long-wave shift, whereas perpendicular excitation results in a high-energy shift.  detail in chapter 2

 The optical properties of such particle arrangements are a complex combination of properties of the particles, of the electronic environment, and of dispersion and arrangement.

Figure 1.4 Eigenmodes of an idealized chain of colloids. (a) longitudinal (b) transverse eigenmode

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1.2.4. Conductivity

4 0 _{e F}

e m V

 





 If the conduction band of metal is only partially occupied by electrons, they can move without resistance in perfect metallic crystal lattice. But they are scattered by the wave character of the electrons.

 The mobility of electrons :

 Mean time between two collisions τ = λ/ U_f

Ex) Copper at RT : U_f = 1.6 x 10⁶ ms^-1, λ = 4.3 x 10^-8m. ∴ τ = 2.7 x 10^-14s

 Scattering mechanisms : 1) dislocation, stacking disorders, ETC

2) scattering at thermal vibrations of lattice (phonon)

 Ohm’s law describes the linear relation between U and I in bulk metal ; U = IR

where, λ = mean free path between collisions m_e= effective mass of the electron

U_f= speed of the electrons (Fermi speed) ε₀= dielectric constant of vacuum

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1.2.4. Conductivity

 The band structure begins to change if the dimension of a metal particle becomes small.

→ Ohm’s law is no longer valid.

→ Single nanoparticles have to be investigated.

 To avoid contact resistance, two capacitances between electrodes and object are used.

Figure 1.4 Experimental conditions for recording a current (I)-voltage (U) characteristic of a single nanoparticle. The capacitances between tips and cluster are formed by the ligand shell

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1.2.4. Conductivity

, C QH

QH

E h

R C



Coulomb 2 U e

  C

 Quantum mechanically describe by the quantum Hall resistance RQH and the capacity of the particle

→ Ref. 2-2

 The current-voltage characteristic for an ideal quantum dot shows no current up to

 Following this an electron tunneling process occurs if the Coulomb energy of the quantum dot is compensated by external voltage of U = ± ne/(2C)

Figure 1.5 I-U characteristic of ideal single electron transport

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1.2.4. Conductivity

FIGURE 1.6 Scanning electron microscopic image of a 17nm ligand-stablized palladium cluster, trapped between two platinum tips. Appl. Phys. Lett., 1997,

71, 1273.

 SEM image of an experiment

 17nm palladium particle is trapped between two PT tips, separated from them by a thin shell of organic molecules(p-H₂N- C₆H₄-SO₃Na).

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1.2.4. Conductivity

FIGURE 1.7 I-U characteristics of the 17nm palladium cluster. At 295K it behaves in a bulklike manner, following Ohm’s law. At 4.2K a pronounced Coulomb blockade is observed.

Appl. Phys. Lett., 1997, 71, 1273.

 At 295K, there is a linear relation between I and U, corresponding to Ohm’s law.

 At 4.2K, a typical Coulomb gap of

~55mV is observed, indicating that the condition kT<<e²/2C is fulfilled.

 To make quantum size behavior of small metal particles available at RT we have to further reduce their size, as the capacitance C of the particles depends directly on their diameter.

 I-U characteristic of the 17nm palladium

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1.2.4. Conductivity

 I-U characteristic of the 1.4nm gold particle

 The gold cluster compound Au₅₅(PPh₃)₁₂Cl₆ shows typical Coulomb blockade even at RT

FIGURE 1.8 I-U characteristics of a ligand-stabilized Au55 cluster at room temperature. Appl. Phys. A. 1998, 66, 189

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1.2.4. Conductivity

 The electronic conductivity of many clusters in a row results from tunneling processes between them through the ligand shells, leading to electronic intercluster bands by resonant tunneling.

 The electronic conductivity of many clusters in a row results from tunneling process, leading to electronic intercluster bands by resonant tunneling.

Idealized cluster conduct electrons without resistance.

FIGURE 1.9 Scheme of the energy levels of a Au₅₅(PPh₃)₁₂Cl₆ cluster. In the ground state the diameter of the 1.4 nm Au₅₅ core corresponds to λ/2 (λ: de Brolie wavelength). The excited state

enables electron tunneling through the ligand shell.

FIGURE 1.10 Ideal cluster wire with tunneling conditions

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1.2.4. Conductivity

 The relationship between distances and activation energies

 If the distances are extended by thicker ligand shells or by spacer molecules, the activation energies increase.

FIGURE 1.11 Correlation between cluster spacing and Coulomb gap of variously ligand Au₅₅clusters.

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1.3.1. Colors

1.3. Applications

 A very obvious property of nanoparticles, at least of some metals, is their specific color.

1) Gold colloids  color glasses

ex) ruby glass : finely dispersed gold colloids

2) silver, copper  attractive colors in glassy materials

 The use of alloys in particular has broadened the availability of nanometals for coloring transparent materials.

FIGURE 1.12

Dispersions of discrete gold nanoparticles in transparent media have an interesting and flexible color gamut that has only

recently been exploited for paints and coatings. These colors depend on how the particles are viewed and on their shape.

The gold particles in the test tubes above are being viewed in transmitted light.

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1.3.2. Catalysis

1.3. Applications

 The increasing proportion of surface atoms with decreasing particle size : small metal particles become highly reactive catalysts

: surface atoms are the active centers for catalytic elementary processes

: the percentage of edge and corner atoms also increases with decreasing size

 Very small metal particles are preferred as catalysts.

 Catalysts based on the activity of metal nanoparticles : the heterogeneous type only

 Industrial process

The particles are generated on supports such as alumina, silica, or charcoal.

 various sizes  larger particles will be less active than smaller ones

 Particles are synthesized not on the support but in separate procedures

 uniform nanoparticles ^{the goal}

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1.3.2. Catalysis

1.3. Applications

FIGURE 1.13

Sketch of a ligand-stabilized metal cluster indicating channels between ligand molecules admit reactants and allow products to leave.

J. Chem. Soc., Dalton Trans., 1996, 5, 589-595.

 Ligand-protected clusters and colloids

: ligand molecules occupy catalytically active sites : tune a catalyst with respect to optimized selectivity : Never cover a particle surface completely

 Enough channels to allow reactants and products to reach the metal surface and to desorb after the catalytic process

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1.3.2. Catalysis

1.3. Applications

 Good catalyst

 enable semihydrogenation

 selectively determine cis- or trans-configuration of the resulting alkene.

 Other possible products(trans-2-hexene, hexane, and olefinic isomers) should be avoided.

well-tuned catalyst

N N

1,10-Phenanthroline

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1.3.2. Catalysis

1.3. Applications

Figure 1.14

(a) Course of the semihydrogenation of hex-2-yne to cis-hex-2-ene with phenanthroline-stabilized palladium clusters as catalysts on TiO₂as a support. (b) The same reaction with 2-butylphenanthroline as ligands on the same palladium clusters. Compared with the reaction in (a) the activity has

decreased dramatically, whereas the selectivity has increased 100%. Formation of other product starts after ~25-30 hours.

J. Chem. Soc., Dalton Trans., 1996, 5, 589-595.

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