Download - Nanoclusters and Nanoparticles
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