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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

KMK 1102

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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KMK 1102

Shape control: Rods of Disks ?+ 30°

+ 20°

+ 10°0°

-10°

-20°

-29°

TiltDirection

KMK 1102

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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KMK 1102

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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KMK 1102

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

KMK 1102

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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KMK 1102

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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KMK 1102

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

kannanmk@u.washington.edu

faculty.washington.edu/kannanmk

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