1
Nanoscopic Materials
• Chemical methods used to change physical and chemical properties –
chemical composition, substituents, concentration, crystal structure….
• Size is another variable to change physical and chemical properties for
constant chemical composition
• Each physical property or fenomenon has a characteristic length
• When particle size is comparable to the characteristic length, property
starts to depend on the size
2
Nanoscopic Scales
Nanomaterials
1 – 100 nm
3
Nanoparticles 1 – 100 nm Traditional materials > 1 m
Nanoscopic Materials
1 nm = 109 m
1 nm = 10 Å
4
Nanoscopic Materials
EU definition (2011):
A natural, incidental or manufactured material containing
particles, in an unbound state or as an aggregate
or as an agglomerate and where, for 50% or more of the
particles in the number size distribution, one or more
external dimensions is in the size range 1 nm – 100 nm.
http://ec.europa.eu/environment/chemicals/nanotech/faq/d
efinition_en.htm
5
Nanoscale regime
Size 1 – 100 nm - Physical and chemical properties depend on
the size !!
Natural examples:
 Human teeth, 1-2 nm fibrils of hydroxyapatite Ca5(PO4)3(OH)
+ collagen
 Asbestos, opals, chalcedony
 Primitive meteorites, 5 nm C or SiC, early age of the Solar
system
Nanoscale objects have been around us, but only now we can
observe them, manipulate, and synthesize them
Nanoscopic Materials
6
Scanning Tunelling Microscopy STM
Binning and Rohrer
Nobel Prize 1986
Nanoscale Writing
STM positioned Xe atoms
on a Ni crystal, 5 nm letters
7
There's Plenty of Room at the Bottom
Richard Feynman
(1918–1988)
NP in Physics 1965
8
Properties of Nanoscopic Materials
• Metallic behavior - a single atom cannot behave as a metal, metal to
nonmetal transition on decreasing the size: 100-1000 atoms
• Magnetic behavior - Single domain particles, large coercive field
• Depression of melting points in nanocrystals
bulk Au mp 1064 C  10 nm Au 550 C
• Negligible light scattering - new optics
• Quantum size effects - information technology, storage media
• High surface area - catalysts, adsorbents
• Large interfacial area - new composites
• Surface modifications - targeted drug delivery, medical and biological
applications
9
Coherence Length, d
XRD patterns of iron oxide nanocrystals
of 4, 6, 8, 9, 10, 11, 12, 13, and 15 nm


cos
k
d 
Scherrer Equation k = 0.89,  = wavelength,
β = full width at half-maximum
(corrected for a natural linewidth
standard, such as Si, LaB6)
10
The Nano-Family
3 dimensions are between 1 - 100 nm
0-D structures (3-D confinement):
• Quantum dots
• Nanoparticles
AFM 1 μm x 1 μm
InAs on GaAs/InP Au nanoparticles
CdTe nanoparticles
11
The Nano-Family
1-D structures (2-D confinement):
• Nanowires
• Nanorods
• Nanotubes
• Nanofibers
At least 2 dimensions are between 1 - 100 nm
12
The Nano-Family
2-D structures (1-D confinement):
• Thin films - CVD, ALD
• Planar quantum wells
• Superlattices
• Graphene
• SAM
At least one dimension is between 1 - 100 nm
Nanoscopic Behavior of Materials
13
• Surface Effects
• Quantum Confinement Effects
Differences between bulk and nanoscale materials
14
Decreasing grain size = Increasing volume fraction of
grain boundaries (50% for 3 nm particles)
Surface Effects
Ru particle
diameter 2.9 nm
15
Surface Effects
Dispersion F = the fraction of
atoms at the surface
F is proportional to surface
area divided by volume
N = total number of atoms
r = radius
V = volume
V ~ r3 ~ N
n = number of atoms at the cube edge
33
2
11
Nrr
r
F 
F
16
Surface Effects
Atoms at surfaces
- Fewer neighbors than atoms in the bulk = lower coordination
number
- Stronger and shorter bonds
- Unsatisfied bonds
- Broad spectrum of interatomic distances and angles
- Surface atoms are less stabilized than bulk atoms
- Reduced atomic density (by 10 – 30 %)
The smaller is a particle, the larger is the fraction of atoms at
the surface, and the higher is the average binding energy per
atom
The melting and other phase transition temperatures scale with
surface-to-volume ratio and with the inverse size 1/r
17
Surface Effects
A = Atoms at surfaces (one layer) – fewer neighbors, lower coordination,
unsatisfied (dangling) bonds
B = Atoms close to surface (several layers) – deformation of
coordination sphere, distorted bond distances and angles
C = Bulk atoms, regular ordering – not present in particles below 2 nm
Graphite shells
18
Si
.
Surface Effects
Experimental evidence
 HR-TEM
 EXAFS, reduced number of nearest and next-nearest neighbors
 Raman spectroscopy
 Mössbauer spectroscopy, quadrupole splitting distribution
broadened
 Diffusivity enhanced by up to 20 orders of magnitude !!
 Solute solubility in the boundary region
Ag (fcc) and Fe (bcc) immiscible in (s) or (l), but do form solid
solution as nanocrystalline alloy
 EPR, nano-Si gives a sharp signal
19
Surface Effects
Calculated mean coordination number <NN> as a function of
inverse radius, represented by N1/3 for Mg clusters
(triangles = icosahedra, squares = decahedra, diamonds = hcp
Mean
coordination
number
What value ?
33
2
11
Nrr
r
F 
20
Surface Effects
Atom binding (vaporization) energies lower in nanoparticles,
fewer neighbors to keep atoms from escaping
Plasticity of nanocrystalline ceramics
Surface Effects in Nanoalloys
21
Alloys:
• Random mixture
• Core-shell
• Janus
Au-Pt 586 atoms
Parameters influencing miscibility
• Atomic size
• Electronegativity
• Surface energy
Transmission Electron Microscopy – Energy
Dispersive X-ray Spectroscopy
22
ICP-OES: Ag 50.3 mol%, EDS: Ag 62.5 mol%
ICP-OES: Ag 68.8 mol%, EDS: Ag 84.2 mol%
Ag@Ni Core-shell NPs
Localized Surface Plasmon Resonance
(LSPR)
23
LSPR = the collective
oscillation of the conduction
electrons on the metallic NPs
excited by the incident photons
at the resonant frequency
coupled to the electromagnetic
field
Metallic NPs with sizes smaller
than the wavelength of light
The resonance frequency of the
oscillation = the surface
plasmon (SP) energy
Localized Surface Plasmon Resonance
(LSPR)
24
The resonance frequency of
the oscillation (LSPR)
• dielectric properties of the
metal
• the surrounding medium
• the particle size
• the particle shape
Effects of Synthesis on Ag-Cu NPs
25
AgCu
413
Ag
393
Cu
569
26
Melting Point Depression
Surface atoms in solids are bound by a lower number of
shorter and stronger bonds
Nanoparticles with a large fraction of surface atoms
• Lowering of average cohesion energy
• Increasing average amplitude of thermal vibrations
• Increasing internal pressure
Result = depression of melting point of nanoparticles
27
Melting Point and Enthalpy Depression
Nanocalorimetry of
Sn nanoparticles
Tm
bulk = 232 C
Hm
bulk = 58.9 J/g
28
Melting Models
Homogeneous Melting Continuous Liquid Melting
Melting particle is
surrounded by liquid
Triple point of coexisting
solid and liquid
nanoparticles of the same
mass surrounded by
vapor
Thin melted layer of a
constant thickness δ
coexisting with solid
core and vapor
Liquid Skin Melting
29
Homogeneous Melting Model
Sn – 4wt%Ag – 0.5wt%Cu
Nano alloy particles
𝑇 𝑟 𝑇 bulk
∆
𝛾 𝛾
Tm(r) = mp of the NPs with radius r
Tm
bulk = mp of the bulk material
 sg = the interfacial energy between the s
and g phases
 lg = the interfacial energy between the l
and g phases
s and l = solid and liquid phase densities
M = molar mass
Hm
bulk = the bulk molar enthalpy of melting
Tm
bulk = 218 C
30
Continuous Liquid Melting Model
rH
V
T
TrT
m
sl
l
mol
bulk
m
bulk
mm


 2)(
Tm(r) = mp of the nanoparticles with radius r
Tm
bulk = mp of the bulk material
Vl
mol = the molar volume of the liquid = M/s
sl = the interfacial energy between the s and l surface
Hm
bulk = the bulk molar enthalpy of melting, endothermic
DSC
Indium NPs confined in pores
Bulk
156.6 C
𝛾 𝛾 𝛾
𝜌 ~𝜌for
Gibbs–Thomson Equation
NPs
31
Phase Transitions
Phase transitions = collective phenomena
With a lower number of atoms in a cluster a
phase transition is less well defined and
broadened
Small clusters behave more like molecules
than as bulk matter
Indium NPs confined in pores
DSC
Bulk
156.6 C
Bulk
156.6 C
First-Order Phase Transitions
32
3 main consequences of a size decrease on caloric curve:
• The transition is shifted, usually to a lower temperature (surface
atoms are less coordinated and less bound than interior atoms)
• The transition temperature is no longer sharp but becomes broad
and takes place over a finite range (fluctuations in TD quantities)
• The latent heat of melting is lower than in the bulk limit
33
Surface Effects on Lattice Constants
Reduction in particle size
• Metal particles usually exhibit a lattice contraction
• Oxide particles exhibit a lattice expansion
YIG = Y3Fe5O12
34
Surface Effects on Lattice Constants
Correlation between the
unit-cell volume (cubic)
and the XRD particle size
in -Fe2O3 nanoparticles
The smaller the particle
size, the larger the unit
cell volume
-Fe2O3
35
Surface Effects on Lattice Constants
The inter-ionic bonding in nanoparticles has a directional character
Ions in the outermost layer of unit cells possess unpaired electronic
orbitals
Associated electric dipole moments, aligned roughly parallel to
each other point outwards from the surface
The repulsive dipolar interactions increase in smaller particles
The repulsive dipolar interactions reduced by allowing unit cell
volume to increase
36
Surface Effects on Lattice Constants
Metal nanocrystals
A continuum elastic model
The lattice contraction
observed in Ag nanoclusters
Interpreted as the result of
hydrostatic pressure exerted
by the surface stress
The surface stress 6.3 N/m
for free Ag NPs 1–7 nm in
diameter
Ag nanoparticles
The smaller the particle size, the
smaller the unit cell volume
37
Quantum Confinement Effects
Physical and chemical properties depend on the size !!
Finite-size effects
MO to Band transition
Quantum Size Effects
38
Band gap dependency
on the nanoparticle size
Fluorescence of CdSe–CdS
core–shell nanoparticles with a
diameter of 1.7 nm (blue) up to
6 nm (red)
Smaller particles have a
wider band gap = blue shift
39
Quantum Size Effects
Metal-to-Insulator Transition
40
Metal-to-Insulator Transition
Metallic behavior
Single atom cannot
behave as a metal
nonmetal to metal
transition 100-1000 atoms
Magnetic behavior
Single domain particles
large coercive field
Band gap increases with decreasing size
Quantum Size Effects
41
Photoelectron spectra of Hg
clusters of nuclearity n
The 6p peak moves
gradually towards the Fermi
level
The band gap shrinks with
increase in cluster size
Hg clusters become metalic
6s
HOMO
6p
LUMOHg
Valence electron
configuration
[Xe] 4f14 5d10 6s2
Quantum Size Effects in Semiconductors
42
b) Wavelength of the absorption
threshold and band gap as a
function of the particle diameter
for various semiconductors
The energy gap in the bulk state in
parenthesis
a) Absorption spectra of CdSe
nanocrystals (at 10 K) of various
diameters
Blue shift
43
Bohr Radii
Quantum confinement - particles must be smaller than the
Bohr radius rB of the electron-hole pair (exciton)
rB = the spatial separation of the electron-hole pair
44
Quantum Confinement Effects
Optical properties
nc-TiO2 is transparent - applications in suncreens
Blue shift in optical spectra of TiO2 nanoparticles
Blue shift
45
Preparation Methods
Top-down: from bulk to nanoparticles
Bottom-up: from atoms to nanoparticles
46
Bottom-up Synthesis: Atom Up
47
Atom Aggregation Methods
GEM – gas evaporation method
 evaporation by heating – resistive, laser, plasma, electron beam,
arc discharge
 the vapor nucleates homogeneously owing to collisions with the
cold gas atoms
 condensation
- in an inert gas (He, Ar, 1 kPa) on a cold finger and walls
metals, intermetallics, alloys, SiC, C60
- in a reactive gas O2 - oxides TiO2, MgO, Al2O3, Cu2O
N2, NH3 - nitrides
- in an organic solvent matrix- metals, carbides
SMAD – the solvated metal atom dispersion
1–2 g of a metal, 100 g of solvent, cooled with liquid N2
more polar solvent (more strongly ligating) gives smaller particles
Ni powder: THF < toluene < pentane = hexan
Carbide formation
77–300 K 450 K
Ni (g) + pentane  NixCyHz  Ni3C
48
Bottom-up Synthesis
Thermal or Sonocative Decomposition of Precursors
Fe(CO)5  nc-Fe + 5 CO sonolytic decomposition
[Co(en)3]WO4  nc-WC – 23% Co thermolysis
PhSi(OEt)3 + Si(OEt)4 + H2O  gel  -SiC (in Ar, 1500 C)
(CH3SiHNH)n (l)  Si3N4 + SiC laser pyrolysis
M(BH4)4 (g)  borides MB2+x (300–400 C, M = Ti, Zr, Hf)
Si(OEt)4 + Ag+ or Cu2+ + H2O  SiO2/Ag+/Cu2+ metal-impregnated gel
SiO2/Ag+/Cu2+ + H2  SiO2/Ag/Cu (550 C)
metal NPs embedded in xerogel
49
Thermal Decomposition of Precursors
Fe(CO)5 oleic acid
trioctylamine
350 o
C, 1 h
350 o
C, 1 h
Fe
Me3NO
Fe2O3
6 nm
Separation of nucleation and growth
Fe(CO)5 thermal decomposition at 100 oC contributes
to nucleation
Fe(oleate) thermal decomposition at 350 oC
contributes to growth
OH
O
Surface Modification
50
A nanoparticle of 5 nm core
diameter with different hydrophobic
ligand molecules both drawn to
scale
The particle is idealized as a
smooth sphere
• trioctylphosphine oxide (TOPO)
• triphenylphosphine (TPP)
• dodecanethiol (DDT)
• tetraoctylammonium bromide
(TOAB)
• oleic acid (OA)
51
Thermal Decomposition of Precursors
200 °C
in dodecylamine
Phase Control
[NnBu4]2[Fe4S4(SPh)4]
pyrrhotite Fe7S8
greigite Fe3S4
thiospinel, the sulfide
analogue of magnetite
180 °C
in octylamine
Thermolysis
52
LaMer Mechanism
Hot-injection synthesis
1) Monomer formation
2) Supersaturated solution
3) Burst of nucleation
4) Depletion of monomer
5) Slow growth of particles without
additional nucleation
Separation of nucleation and
growth - monodisperse
2
1
3 4
5
53
Other Mechanisms
Digestive Rippening
The conversion of polydisperse NPs into monodisperse ones
The etching of large NPs by dissolution of clusters/atoms by digestive ripening
agents - strongly coordinating ligands
Clusters/atoms redeposited on small NPs = the growth of smaller NPs
Narrowing of the particle size distribution = monodisperse system
A thermodynamic equilibrium size of the NPs is usually obtained
Depends on the specific ligand and the reaction temperature
Watzky-Finke Mechanism
Slow continuous nucleation - Fast autocatalytic surface growth
Seed-mediated Mechanism
Au nanoclusters as seeds - Bi, Sn, In, Au, Fe, Fe3O4
54
Borohydride Reduction
Reduction of Metal Ions
Manhattan Project
Aqueous, under Ar
2 Co2+ + 4 BH4
- + 9 H2O  Co2B + 12.5 H2 + 3 B(OH)3
Under air
4 Co2B + 3 O2  8 Co + 2 B2O3
Nonaqueous
Co2+ + BH4
- + diglyme  Co + H2 + B2H6
TiCl4 + 2 NaBH4  TiB2 + 2 NaCl + 2 HCl + H2
MXn + n NR4[BEt3H]  M + NR4X + n BEt3 + n/2 H2
M = group 6 to 11, Bi, Sn, ; n = 2,3; X = Cl, Br, NO3, OAc, OOC-R, acac, O-R
Solvents: Diethylenglycol, Oleylamine, ….
Surfactant
Mixed-metal particles AgNi, AgCu, BiNi, ….
NaBH4
BH3NH2tBu
NR4[BEt3H]
Ni‐Sn alloy
55
Borohydride Reduction
SH
Two-dimensional array of
thiol-derivatised Au NPs
(mean diam 4.2 nm)
Au colloidal particles
HAuCl4 + NaBH4 in toluene/H2O system, TOABr as
a phase transfer agent, Au particles in the toluene
layer, their surface covered with Br, addition of
RSH gives stable Au colloid
56
K +
K+
Mg
Mg
Solvents: dry anaerobic diglyme, THF, ethers, xylene
NiCl2 + 2 K  Ni + 2 KCl
AlCl3 + 3 K  Al + 3 KCl
Reduction by Glycols or Hydrazine
“Organically solvated metals”
Alkali Metal Reduction
57
Alkalide Reduction
13 K+(15-crown-5)2Na + 6 FeCl3 + 2 CBr4
THF
30 °C
2 Fe3C (nano) + 13 K(15-crown-5)2Cl0.43Br0.57 + 13 NaCl
Anealed at 950 °C / 4 h
Fe3C: 2 – 15 nm
58
Reactions in Porous Solids
Zeolites, Mesoporous materials
Ion exchange in solution, reaction with a gaseous reagent
inside the cavities:
M2+ + Na-Y  M-Y + 2 Na+
M2+ + H2E  ME M = Cd, Pb; E = S, Se
Ship-in-the-Bottle Synthesis
Ru3+ + Na-Y  Ru(III)-Y + 3 Na+
Ru(III)-Y + 3 bpy  Ru(bpy)3
2+ reduction of Ru(III)
Conducting carbon nanowires
Acrylonitrile introduced into MCM-41 (3 nm diam. channels)
Radical polymerization
Pyrolysis gives carbon filaments
59
Bottom-up Synthesis
Sol-Gel Methods
Sol drying
Aerogels, supercritical drying
Aerosol Spray Pyrolysis
Aqueous solution, nebulization,
droplet flow, solvent evaporation,
chemical reaction, particle
consolidation, up to 800 C
3 Gd(NO3)3 + 5 Fe(NO3)3  Gd3Fe5O12 + 6 O2 + 24 NO2
MnCl2 + 2 FeCl3 + 4 H2O  MnFe2O4 + 8 HCl
60
H2O +
octane
H2O
H2O
Cd2+ Se2-
CdSe
CdSe
PhSeSiMe3
Bottom-up Synthesis
Inverse micelles
Size distribution histogram
61
Rapid Expansion of Supercritical Fluid
Solution
Polymeric
Nanoparticles
Polymer
in CO2
62
Spinning Disc Processing SDP
A rapidly rotating disc (300-3000 rpm)
Ethanolic solutions of Zn(NO3)2 and NaOH, polyvinylpyrrolidone (PVP)
as a capping agent
Very thin films of fluid (1 to 200 m) on a surface
Synthetic parameters = temperature, flow rate, disc speed, surface
texture
Influence on the reaction kinetics and particle size
Intense mixing, accelerates nucleation and growth,
affords monodispersed ZnO nanoparticles with controlled particle size
down to a size of 1.3 nm and polydispersities of 10%
63
Electrospinning
ThO2 from PVA/Th(NO3)4
Average diameter: 76 ± 25 nm
Parameters
• Solution – precursor + polymer + solvent
(viscosity, conductivity, surface tension)
• Instruments (voltage, distance b/w electrodes,
collector shape)
• Ambient (temperature, humidity, atmosphere)
Vapor-Liquid-Solid (VLS) Growth
64
Synthesis of nanowires NW
Metal catalyst nanoparticles - Au(s) – (1)
Feed another element (Ge vapor, GeH4 or SiH4)
at an elevated temperature (440-800 C/ultra-
high-vacuum)
Gaseous precursor feedstock is
absorbed/dissolved in Au(s) till the solid
solubility limit is reached (2)
A liquid phase appears (3), melts to a droplet
The droplet becomes supersaturated with Ge
When the solubility limit is reached (4), an
excess material is precipitated out to form solid
NWs beneath the droplet
Eutectic 360 C
Au (mp 1064 C)
Si (mp 1410 C)
Ge (mp 938 C)
Vapor-Liquid-Solid (VLS) Growth
65
1
2 3
4
66
Vapor-Liquid-Solid (VLS) Growth
Au NPs
Au/Ge Eutectic
Liquid Alloy
Nucleation of NWs
NW Growth
1
2 3
4
1
2 3 4
GeH4
67
In-situ TEM images of the VLS process
In-situ TEM images recorded during
the process of nanowire growth:
(A) Au nanoclusters in solid state at
500 °C
(B) Alloying initiated at 800 °C, at this
stage Au exists mostly in solid state
(C) Liquid Au/Ge alloy
(D) The nucleation of Ge nanocrystal
on the alloy surface
(E) Ge nanocrystal elongates with
further Ge condensation
(F) Ge forms a wire
Top-down Synthesis: Bulk Down
68
 Introduction of Crystal Defects (Dislocations, Grain Boundaries)
• High-Energy Ball Milling - final size only down to 100 nm
(contamination issues)
• Extrusion, Shear, Wear
• High-Energy Irradiation
• Detonative Treatment
 Crystallization from Unstable States of Condensed Matter
• Crystallization from Glasses
• Precipitation from Supersaturated Solid or Liquid Solutions
69
Top-down Synthesis: Bulk Down
Lithographic Techniques
electron beam and focused ion beam (FIB) lithography
Top-down Synthesis: Bulk Down
70
Lithographic Techniques
electron beam and focused ion beam (FIB) lithography