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KMK 1102
Kannan M. KrishnanKannan M. KrishnanYuping BaoYuping Bao
Mike Mike BeermanBeerman
Department of Materials Science Department of Materials Science University of Washington, SeattleUniversity of Washington, Seattle
NSF/DMRNSF/DMRCampbell Endowment at UWCampbell Endowment at UW
UW-PNNL Joint Institute of UW-PNNL Joint Institute of NanoscienceNanoscience
Invited talk: American Vacuum Society, Denver (11/02)
KMK 1102
Outline
Magnetic properties of NC arrays
Preparation of Nanocrystals Passivation of Nanocrystals Obtaining Narrow Size Distributions Characterization Controlling the Shape of Nanocrystals
Self-Assembly of Nanocrystals Entropy as the driving force Experimental Variables: size, shape & magnetic interactions
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KMK 1102
Isolation and Purification of NanocrystalsElectrostatic Stabilization
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Isolation and Purification of NanocrystalsSteric Stabilization
Barrier to aggregation is proportionalto energy of mixing between the tethered capping group and solvent
Capping groups provide surfacepassivation (covalently boundligands) & sufficient repulsion.
Engineering Interparticle Separation TBPO < TOPO < THPO
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KMK 1102
Preparation of Colloidal Metals in Constrained Environments
Notes:Separate nucleation from growthTemporally discrete nucleationeventSlow controlled growth on existingnuclei
La Mer & Dinegar, J.Am. Chem. Soc. (1950)
Murray, Kagan and Bawendi, Ann. Rev. Mat. Sci.(2000)
KMK 1102
Details of Co Synthesis
Cobalt Carbonyl (0.5g) + Dichlorobenzene (3 ml)
Trioctylphosphene oxide (TOPO) +Oleic Acid + DCB0.1-0.2 g 0.2 ml 12 ml
Inject (rate & temperature) into
Centrifuge
Nanocrystal Particles
Disperse in solvents
SELF ASSEMBLY
*critical parameters
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KMK 1102
Issues in La Mer synthesis
� Reaction time - controls particle size� Injection (temperature and rate) - controls nucleation and hence particle size distribution� Surfactant� Solvent (s)� Concentration of metal precursor, surfactant
� Control of shape� Alloying� Control and/or prevention of oxidation� Control of interparticle separation� Self-assembly
KMK 1102
30 40 50 60 70 80
CO
UN
TS
(ar
b. u
.)
DEG
ε-Cobalt (11 nm particles)
(211)
(310)
(311) (510)
(420)(321)
6.09 Å
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KMK 1102
(15”) (100”)
(300”) (1000”)
Kinetic Control of Nanocrystals Shape ?
40 50 60 70
60
75
90
105
120
135
150
-002
-100 -1
01
Cou
nts
2θ
Cohcp
40 50 60 700
200
400
600
800
1000
-330
-311
-310
-221
Cou
nts
2θ
Coεεεε
KMK 1102
Selective bonding of surfactants to specific Co surfaces ?
Oleic Acid R C
O
O
H
TOPO P
RRR
O
Oleyl Amine R N
H
H
Thiol R S
H
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KMK 1102
TOPO
OleicAcid
Plates ?
OleicAcid
OleylAmine
Rods ?
Nanocrystal shape control with multiple surfactants ?
KMK 1102
Nanocrystals shape control: the role of surfactants
Oleic acid Tri-n-octylphosphine oxide Oleyl amine
1:1 1:1.6 1:2.7
Plates or rods ?
Rods or Disks ?
Spheres
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Shape control: Rods of Disks ?+ 30°
+ 20°
+ 10°0°
-10°
-20°
-29°
TiltDirection
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General Requirements for Shape Controlled Synthesis
� Suitable organo-metallic precursor that rapidlydecomposes to yield monomers at temperatureswhere the surfactants are stable.
� Two surfactants must be found thatdifferentially adsorb to the growing particlesurface leading to rod formation.
� One surfactant must promote monomerexchange between particles to allow for sizedistribution focussing.
Puntes, Krishnan & Alivisatos, Science, 291, 2115 (2001); Bao, Beerman & Krishnan (unpublished)
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KMK 1102
Self Assemby of Colloidal Nanocrystals
� Under appropriate conditions particles is suspension spontaneously self-assemble
�First order Fluid -> Solid phase transition
�To control the structure of the colloid “crystalline” phase need to a) Control interaction among particles b) Control particle Kinetics
�ENTROPY plays an important role in this spontaneous self-assembly.
KMK 1102
Self Assemby: Entropy as the Driving Force
(Phase Transitions in Hard Sphere and/or charge-stabilized colloids)
Hard Materials
Thermodynamic EquilibriumMinimize Gibbs Free Energy
F = E - TS
E >> TS
i.e. internal energy determinesequilibrium phase and thermal
fluctuations are treated asperturbations.
Soft Materials (Colloids)
“Hard Sphere” systems (weak repulsive potential)
E << TS
Free energy determined by entropy (S) which is afunction of the packing fraction, φ
S (φ)
As volume fraction increases, particle motion isrestricted by collisions
Freezing (first order phase transition)
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Self Assemby: Entropy Driven First-Order Phase Transformation
BernalGlass
RandomClose-pack
0.494
Freezing
0.545
Melting
0.64 0.74
hcp
100%
0% φ
Vol%Crystal
Remarkable FeatureTwo observed close-packeddensities.
Φr
Φh
Note : Φh > Φr
Φh ~ 0.74
Φr ~ 0.64
As φ , S drives FO transformation
KMK 1102
Size-dependent magnetic behavior of Co nanocrystals
Properties and magnetic characteristic lengths of the 3d transition-metal ferromagnetMs Ku J Tc lH lK lS Dcrit Ds
emu/cc erg/cc erg/cc °C nm nm nm nm nm
Fe 1714 8x105 1.7x10-6 770 14.5 17.5 3.5 14 16.0Co 1422 7x106 2.2x10-6 1131 17.5 5.5 4.2 70 7.6Ni 484 5x104 1.0x10-6 358 19.5 45.0 8.0 55 39.1
~
Co Spheres 6 nm
-0.06
-0.04
-0.02
0
0.02
0.04
0.06
-6000 -4000 -2000 0 2000 4000 6000
Applied Field (Oe)
Mo
men
t (e
mu
)
Co Spheres 20 nm
-0.5
-0.4
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
0.4
0.5
-2500 -2000 -1500 -1000 -500 0 500 1000 1500 2000 2500
Applied Field (Oe)
Mo
men
t (e
mu
)
thermalprocess
singledomainrotation
singledomaincurlingmode
domainwallmotion
superparamagnetic
single domain multi - domain
defects act asnucleation sites
defects act aspinning sites
perfectcrystal
defectivecrystal
grain size
Hc
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Self-assembly of intermediate size (8-10 nm) Co nanocrystals (superparamagnetic)
Classical Entropy-driven 1st Order Phase Transition
Co Spheres 6 nm
-0.06
-0.04
-0.02
0
0.02
0.04
0.06
-6000 -4000 -2000 0 2000 4000 6000
Applied Field (Oe)
Mo
men
t (e
mu
)
40 50 60 700
200
400
600
800
1000
-330
-311
-310
-221
Cou
nts
2θ
Coεεεε
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Self-assembly of very small (3-5 nm) Co nanocrystals
40 50 60 700
200
400
600
800
1000
Coεεεε
Tentative Model(s): Steric Forces Dominate
Co Spheres 6 nm
-0.06
-0.04
-0.02
0
0.02
0.04
0.06
-6000 -4000 -2000 0 2000 4000 6000
Applied Field (Oe)
Mo
men
t (e
mu
)
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Projection of ParticlesIn Special Orientations(652)
V
dZL Wang et al, 2001
Working Hypotheses
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Self-assembly of Bimodal size distributions
Two different NC SizesRole of Surfaces
Entropy-induced Wetting: Depletion forces determine self-assembly
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Self Assembly of Co Nanoparticles: Large FM particles
40 50 60 700
200
400
600
800
1000
-330
-311
-310
-221
Cou
nts
2θ
Coεεεε
Magnetostatic interactions dominate self-assembly
Co Spheres 20 nm
-0.5
-0.4
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
0.4
0.5
-2500 -2000 -1500 -1000 -500 0 500 1000 1500 2000 2500
Applied Field (Oe)
Mo
men
t (e
mu
)
KMK 1102
Self Assembly of Co Nanoparticles: Disks
40 50 60 70
60
75
90
105
120
135
150
Cohcp
-1-0.8-0.6-0.4-0.2
00.20.40.60.81
-2000 -1000 0 1000 2000
Applied Field (Oe)
Magnetostatic interactions dominate self-assembly
0.2 µm
5 x 20 nm Disks
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KMK 1102
Self-assembly of NCs: Solvent-Nonsolvent Pair Precipitation
� Efficiency of steric stabilization strongly dependent on interaction of alkyl group (surfactant) with the solvent.
� Gradual addition of a non-solvent or the evaporation of a solvent from a solvent-nonsolvent mixture can produce size-dependent flocculation
Er
Van der Waals (r-6)
Steric repulsion (r-12)
Methanol + Hexane Methanol + Toloune Toloune + Hexane
KMK 1102
Dispersion on Surfaces: Solvent Surface Tension
High Surface Tension (DCB)
Low Surface Tension (Hexane)
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KMK 1102
Self Assembly of Co Nanoparticles
Second Monolayer
First Monolayer
40 nm
KMK 1102
Size-dependent magnetic behavior of Co nanocrystals
Properties and magnetic characteristic lengths of the 3d transition-metal ferromagnetMs Ku J Tc lH lK lS Dcrit Ds
emu/cc erg/cc erg/cc °C nm nm nm nm nm
Fe 1714 8x105 1.7x10-6 770 14.5 17.5 3.5 14 16.0Co 1422 7x106 2.2x10-6 1131 17.5 5.5 4.2 70 7.6Ni 484 5x104 1.0x10-6 358 19.5 45.0 8.0 55 39.1
~
Co Spheres 6 nm
-0.06
-0.04
-0.02
0
0.02
0.04
0.06
-6000 -4000 -2000 0 2000 4000 6000
Applied Field (Oe)
Mo
men
t (e
mu
)
Co Spheres 20 nm
-0.5
-0.4
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
0.4
0.5
-2500 -2000 -1500 -1000 -500 0 500 1000 1500 2000 2500
Applied Field (Oe)
Mo
men
t (e
mu
)
thermalprocess
singledomainrotation
singledomaincurlingmode
domainwallmotion
superparamagnetic
single domain multi - domain
defects act asnucleation sites
defects act aspinning sites
perfectcrystal
defectivecrystal
grain size
Hc
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100 150 200 250 300
0.0
0.2
0.4
0.6
0.8
1.0
χχχχ'
T(K)
HH H
t=1 t=2 t=3
KV>27kBT KV~27kBT KV<27kBT
Low Field
27kBT
High Field
t=1 t=2 t=3
Zero Field Cooled
KMK 1102
HH H
t=1 t=2 t=3
KV>27kBT KV~27kBT KV<27kBT
Low Field
27kBT
High Field
t=1 t=2 t=30 50 100 150 200 250
0
4
M(e
mu
*10
-5)
T (K)
Cooled Under Field
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KMK 1102
Magnetic Behavior of Co Nanocrystal Arrays: ZFC & FC
0 100 200 300
M (
arb.
u.)
T (K)
FC
ZFC
TB
0 50 100 150 200 2500
2
4
6
8
10
12
14
M(e
mu
10
-6
)
T (K)
FC
ZFC
50 nm
0 25 50 75 100 125 150 175 200 225 250 275
2
4
6
8
10
M(e
mu
10-
6)
ZFC
FC
IdealNon-interactingSuperparamagneticNanocrystals(monodisperse)
KMK 1102
Tc =µo µ2
4 π kB d3
µo
4 π kB
µ Ms φ~
Dipolar Ferromagnets ?Ferromagnetism in the absence of exchange interactions
Ground State
SCLColumnar
Antiferromagnet
FCC /BCC
DipolarFerromagnet ?
Luttinger and Tisza (1946)Roser and Corrunccini (1990)Bouchaud and Zerah (1993)
µµµµ ~ 1-2 µµµµB Tc ~ mK
µµµµ > 3000µµµµB Tc ~ ??
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KMK 1102
Work in progress: ZFC - FC- TRM measurements
ZFC - FC - TRM
0
0.002
0.004
0.006
0.008
0.01
0.012
0 50 100 150 200
Temperature (K)
Mo
men
t (e
mu
)
ZFCFCTRM
Collaborators: Per Norbladt and Petra Jonsson, Uppsala University
KMK 1102
Work in progress: ZFC - MEM measurements
0.00.20.40.60.81.01.21.41.6
0 25 50 75 100 125 150 175 200
Temperature (K)
Mo
men
t (a
rbit
rary
)
ZFC
MEM
-0.004-0.0020.0000.0020.0040.0060.0080.0100.0120.014
0 25 50 75 100 125 150 175 200
Temperature (K)
Mo
men
t D
iffe
ren
ce
Collaborators: Per Norbladt and Petra Jonsson, Uppsala University
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KMK 1102
Summary
Magnetic properties of NC arrays
Preparation of Nanocrystals Passivation of Nanocrystals Obtaining Narrow Size Distributions Characterization Controlling the Shape of Nanocrystals
Self-Assembly of Nanocrystals Entropy as the driving force Experimental Variables: size, shape & magnetic interactions
KMK 1102
faculty.washington.edu/kannanmk
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