NanomaterialsNanomaterials

The family of nanomaterials

� Atom clusters, nanoparticles, quantum dots…

� Thin films

� Nanotubes, rods, belts, spheres…

� Dendrimers, supra molecular structures, biomolecules…

ZnO tubeZnO belt TiO2 spheres

SWCNT

Co-Pc

dendrimer DNA on micacatenane

Alkane thiol self-assembly on Au

Tunnel spin-valve head

C60

FePt-Al-O

QD in the nucleus of a cell

Nanometer clusters and

particles

Nanometer clusters and

particles

�Nanoparticles: are the simplest nano-structures. In principle, any assembly of atoms bonded together with a radius of < 100 nm can be considered a nanoparticle.

� These can include:

� Fullerenes

� Metal clusters (agglomerates of metal atoms)

�Oxide (magnetic) and semiconducting nanoparticles (QD)

�Large molecules, such as proteins, and supramolecular structures

1. Nanoparticles

< 100 nm

C60

Au

QDFe2O3

Ferritin

Nan

oclu

ster

sN

anoc

lust

ers

� Free atomic clusters: constituted by no more than a few hundred of atoms (maximum size of a few nm).

1.1 Nanoclusters

C60Kr clusters

FePt icosahedralcluster

�Nanocluster: is a nanometer sized particle made up of equal subunits (condensed hard matter). These subunits can be atoms of a single element, molecules, combinations of atoms of several elements in subunits of equal stoichiometries (alloys) …

E.g.: Nan, (Cu3Au)n, (H2O)n, (TiO2)n …

�Molecules have functionality which depends on the inter-positioning

of their atoms, whereas the properties of nanoclusters are solely

governed by the number of subunits they contain.

�In general, the physical properties of materials are dependent on the

size and are scaleable with the amount of atoms

- scaleable size regime - →

1.1 Nanoclusters

N γ−

�Non-scaleable size-regime: Every atom counts!� Nanoclusters → their properties vary greatly with

every addition or subtraction of an atom

�Size does matter!

1.1 Nanoclusters

Small is very different !

Small is very different !

� Crystal structure of large nanoclusters (>1000 atoms): the same as the bulk structure of the material with somewhat different lattice parameters (clusters are slightly contracted).

E.g.: Cu clusters tend to have an FCC structure

� Smaller clusters of metallic atoms have other atomic arrangements which minimize surface energy (and internal energy).

1.1 Nanoclusters: geometric structure

Wulff polyhedron

Closed geometric shells

� Very many atoms are sitting on the surface of a cluster:large effect of the surface energy.

� Structural magic numbers: closed geometric shells occur for icosahedral geometry (13, 55, 147, 309, 561…).�Five-fold symmetry cannot occur in bulk materials!

� Examples of clusters with icosahedral geometry.

1.1 Nanoclusters: geometric structure

561 Au Ag icosahedral W-Au12

� Structural magic numbers: occur when an exact amount of atoms is needed for a specific structure. Usually these species are more abundant than the rest.

� Smaller clusters can be also amorphous-like (disordered), spherical, …

1.1 Nanoclusters: geometric structure

tetrahedral Au20

5 nm Au

Au55 Ag75

icosahedral Au147

� Structural magic numbers affect the formation and abundances of larger clusters and noble gas clusters, whereas for smaller metallic clusters the combined electronic structure of all the atoms is of greater importance.

The jellium model� It envisions the cluster as a single large atom, where the distribution of

ionic cores is replaced by a deformable positive background (jellium

density), and only valence electrons are treated explicitly.

Energy levels can be calculated by solving the Schrödinger equation in

a similar manner to that for the hydrogen atom.

1.1 Nanoclusters: electronic structure

Deformable spherical

potential well

atom cluster

1s2 1s2

1p61d102s21f14

2s22p63s23p6

� Experimental abundance spectrum for sodium clusters compared with the ionization potentials calculated by the Clemenger-Nilsson model. In this case, electronic structure rather than geometrical factors governs the stability of the clusters.

W. D. Knight et al., Phys. Rev. Lett. 52 (1984) 2141

�The maxima (2, 8, 20) can be associated with the fully filled-up shells

in a spherical potential (s, s+p, s+p+d+s). Compare these numbers to

the electronic level filling in the periodic table and the related

chemical inertness of elements.

1.1 Nanoclusters: electronic structure

� When a bulk lattice is formed for larger metallic clusters, the discrete energy levels are grouped tending to form energy bands. However, for small sizes these levels are still very different to the continuousbands of bulk materials.

�Many properties of the material are dramatically modified! E.g.:

optical and electrical transport properties.

1.1 Nanoclusters: electronic structure

E E E

size

� In semiconductors the band gap will be increased as particle size is decreased (blue shift in the absorption spectrum of the semiconductor). Energy level separations are also dependent on the size of the clusters, which affects the energies needed for the transitions of electrons to excited states.

�The fluorescence spectrum

depends on the size!

1.1 Nanoclusters: optical properties

1.9 – 6.7 nm

Fluorescence of cadmium selenide QD’s

D. Talapin, University of Hamburg, Physica E 17, 99 (2003)

� Since the electronic structure of nanoclusters depends on size, their ability to react with other species should also depend on size. Reactivity is highly dependent on the electronic structure, leading to large variations even for sizes differing only by a single atom.

E.g.: gold nanoclusters are highly reactive if compared to the fairly inert bulk material

1.1 Nanoclusters: reactivity

Nanoclusters: higher surface to

volume ratio

Higher reactivityHigher

reactivity

� Lindermann (1906): a solid melts if its thermal fluctuations become too large; the solid shakes itself apart.

Melting starts at the surface!

1.1 Nanoclusters: melting point

� For large gold clusters the melting temperature is reduced as 1/(cluster radius).

1.1 Nanoclusters: melting temperature

Melting starts at the

surface!

Melting starts at the

surface!Koga et al. PRL92 11507 (2004)

�Melting temperature for Na clusters: more complex dependence for smaller sizes.

1.1 Nanoclusters: melting temperature

Solid-to-solid transition between N=400 to 1000

�Negative heat capacities have been observed in nanoclusters under certain conditions. At the melting temperature, an increase of the internal energy of by 1 eV leads to a decrease in temperature by about 10 K.

� In large enough systems added energy is converted completely into

potential energy, reducing continuously the fraction of its solid phase.

The kinetic energy and thus the temperature remain constant. A small

system tries to avoid partially molten states and prefers to convert some

of its kinetic energy into potential energy instead.

1.1 Nanoclusters: negative heat capacity

The cluster can become colder, while

its total energy increases

The cluster can become colder, while

its total energy increases

+

147Na

1.1 Nanoclusters: negative heat capacity

A micro-canonical system can have

negative heat capacity

A micro-canonical system can have

negative heat capacity

H. Haberland et al., PRL86 1191 (2001)

� The cluster has a net magnetic moment: the interatomicmagnetic interactions can force all the atomic moments to align in one direction with respect to some symmetry axis of the cluster (it will be magnetized).

1.1 Nanoclusters: magnetism

The cluster behaves like a small magnet

The cluster behaves like a small magnet

M

Nanoparticulatematerials

Nanoparticulatematerials

�Naked nanoparticles: (metallic, oxidized, semiconducting …) in the form of a powder (size between ≈ 2-100 nm).

1.2 Nanoparticulate materials

Co 9nm Fe2O3 Ag nanoprisms

7 nm CdSe (QD)

10 nmTetrapod of CdSe

�Nanoparticles embedded in a matrix: (metallic, oxidized, semiconducting …) in the form of a thin film or a bulk material.

1.2 Nanoparticulate materials

ZnO in Al-O matrix FePt in Al-O matrix

Co in ZrO2 matrix

�Nanoparticles in a colloidal suspension: metallic, oxidized, semiconducting … Usually they are coated by a surfactant which avoids particle agglomeration.

� Stability?

1.2 Nanoparticulate materials

Length scale: macroscopic colloid nanoparticle

� Sedimentation small particles (≈10 nm)

� Agglomeration surfactant (e.g. oleic acid)

Surf ace act ive agent : Organic compounds containing both hydrophobic and hydrophilic groups.

� Bulk nanocrystalline materials (nanocomposites): They can be prepared by slow deposition or by consolidation of nanocrystalline powders.

1.2 Nanoparticulate materials

Nanocomposite of CNbnanocrystalls in an

amorphous C matrix

Nanocrystalline structure in a copper thin film

� Bulk materials:�Metals are usually considered to be ductile and malleable.�Ceramics are usually considered to be elastically hard and

brittle.�Grain sizes move into the nanoscale (simplified view):

�Metals get stronger and harder and more brittle.�Ceramics become more ductile, loosing elastic hardness

This is, however, a simplification: reality is more complex!

1.2.1 Mechanical properties

One can improve on the properties of both metals and ceramics

towards the other class of materials

One can improve on the properties of both metals and ceramics

towards the other class of materials

Nanocrystallinematerials

� The yield strength σy: Point in the stress vs. strain curve (strain is relative elongation) where an external force has lead a permanent deformation of 0.2% after the external pressure is relieved. It is a measure of how much you can draw out a material before it fails for practical purposes.

The plastic behavior is usually controlled by the motion of dislocations.

1.2.1 Background: yield strength

655565Molybdenum

330240Titanium

480138Nickel

262130Iron

6928Copper

130----------Gold

Tensile

strength

(MPa)

Yield

strength

(MPa)

Metal

�Hall and Petch (1950): they found that σy of a polycrystalline metal follows:

where σ0 and K are constants depending on the specific material.

� Explanation: the grain boundaries hinder dislocation motion, thereby making plastic deformation more difficult at small grain sizes.

Many polycrystalline metals obey such a relationship over several orders of magnitude in grain size!

1.2.1 Background: Hall-Petch relation

0y

K

dσ σ= +

� For nanoscale materials:

which is obviously not true.

�Natural lower limit: atom size (in this limit the H-P relation is clearly not valid).

�Critical size: minimum grain size for which at least one dislocation loop must fit into average grain.

Hall-Petch relationship breaks down below a critical size!

1.2.1 Breakdown of Hall-Petch relation

0 yd σ→ ⇒ → ∞

0.2 nmd ∼

� For nanometer grain sizes atomistic simulations of the deformation (molecular dynamics).

1.2.1 Reverse Hall-Petch effect

�MDS in nanocrystalline copper

� Flow stress: average stress in the strain interval 7 to 10%

Reverse H-P effect

J. Schiotz, Science 301, 1357 (2004)

�Deformation at the grain scale.(A) d=49 nm after 10% deformation. (B) additional strain accumulated when deformation is increased from 10% to 11%. (C and D) the same for d=7 nm.

� At small sizes: deformation is mainly located at the boundaries and is mediated by atomic sliding.

1.2.1 Reverse Hall-Petch effect

J. Schiotz, Science 301, 1357 (2004)

Scale bars: 5 nm

�Cu of 50 µm: σy=56 MPa.

�Cu of 23 nm: σy =770 MPa

�Good ductility: general rule (metals becoming brittle as the size of the grain is reduced) is not always true!

1.2.1 Hall-Petch effect in Cu nanocrystals

Youssef, APL85, 929 (2004)

A truly dramatic improvement of

strength

A truly dramatic improvement of

strength

Mechanical milling in situ

consolidation