Solution Phase Routes to Transition Solution Phase Routes to Transition Metal Doped TiOMetal Doped TiO22

J. Daniel BryanGamelin Research Group

July 10th 2003

Spintronics Symposium

The Challenge

Cobalt enrichment & Cluster formation

Chambers, S. et al, App Phys Lett (2003), 82, 8, 1257

Why Alternate Routes to Diluted Why Alternate Routes to Diluted Magnetic Semiconductors (Magnetic Semiconductors (DMSsDMSs)) ??

• Wet chemical control of oxdiation states

• Ability to template novel architectures using solution phase precursors

• Study magnetic exchange at various length scales: nano,meso, micro

• Low cost

NanocrystalNanocrystal DMS DMS Length ScalesLength Scales

DMS nanocrystal

< 10nm

nano

electronically coupledcluster of nanocrystal

meso

continuous film of DMS nanocrystals

micro

Colloids Clusters Films10-400 nm > 400 nm

SolSol--Gel BasicsGel Basics“Sol” – A stable suspension of colloidal particles within a liquid (2 –200 nm)

“Gel” – A porous 3-dimensional interconnected solid network that expands in a stable fashion throughout a liquid medium and is limited by the size of the container.

nanoparticle systems via “Sol- Gel” route means hydrolysis & condensation mechanism We totally avoid the Gel step.

Co2+

Co2+Co2+

Co2+

Co2+

Co2+Co2+

Co2+Co2+

Ti4+

Co2+

TiO2

Co2+

Co2+

Co2+

Co2+

hydrolysis

condensation

Solution Phase Routes to TiOSolution Phase Routes to TiO22 NanocrystalsNanocrystals

• Non-hydrolytic lyothermal – (Colvin et al.)– elimination of surface Ti-OH (traps, less strongly

reducing) – elimination of surface bound H2O

• Catalyzed Sol-Gel (hydrolytic)• Acid/Base/Water (many authors)

• Inverse micellular Sol-Gel (Lin et al., Bryan et al.)– Pseudo-nonhydrolytic– Controlled hydrolysis

Lyothermal Lyothermal NonNon--Hydrolytic RouteHydrolytic Route

TiX4(l) + Ti(OR)4(l) 2TiO2(s) + 4RXTOPO, airfree

300 °C,5 min

5 nm particlespros: highly crystalline, rapid cons: difficult to dope, transition metal precursors difficult

“etherolysis”M(OR)z-1

R

•O•••

δ+MX

X

X

X

-M-O M (OR)z-1 + RXX

X

X

2) Condensation

oxolation

-M-O M OR

••••+H

••••

O

H

H•••• + M(OR)z

O

H

H••••

δ-

M(OR)z-1

R

•O•••

δ+

leaving group

+

ROH

proton rearrangement

-M-O M HORδ+

+

1) Hydrolysis

M(OR)z-1

δ+

•O••• H

How to Create a TiHow to Create a Tiδδ++--OO--CoCoδδ++ Bond Bond Part I:Part I: “Partial Charge Model”“Partial Charge Model”

δ+

Species δ(M) N N-zSi(Oet)4 0.32 4 0

Ti(Oet)4 0.63 6 2

δ (M) ”partial charge” is related to the Pauling electronegativity

greatly influences hydrolysis rates, Ti is muchfaster than Si, can’t get a mixed Si/Ti species by normal Sol-Gelin excess H2O.

How to Create a TiHow to Create a Tiδδ++--OO--CoCoδδ++ Bond Bond Part II: Limiting the WaterPart II: Limiting the Water

“AOT”

sodium bis(2-ethylhexyl) sulfosuccinate

( )( )surfactmoles

OHmolesW−−

= 20

• at low W0, little or no free unbound H2O.•at high Wo precipitation occurs

• limited hydrolytic route to TiO2

W0=4.5, rw= 1.4 nm, rh=2.3 nm*

*Maitra, A. J Phys Chem, (1984), 88, 5122

inverse Micelles have:surfactant, water, non-polar phase

Co2+

Co2+

Co2+Co2+

Co2+

Inverse Micelle PreparationInverse Micelle PreparationDoping During HydrolysisDoping During Hydrolysis

Inverse Micelle + Cobalt

Ti precursor/co-surfactant

air-free transfer

exposed to air

light purple to light blue

light rose to light purple

stir

teflon linedAutoclaveambient atmospheric overhead (25 C)

~125-150 °C24 hours

colloidal dopednanocrystals

Spectroscopic Characterization of SynthesisSpectroscopic Characterization of Synthesis

0.4

0.2

Abso

rban

ce (a

.u.)

30 25 20 15Energy (103 cm-1)

x 10

Surface Chemistry Surface Chemistry -- Cobalt SpeciationCobalt SpeciationInternal & SurfaceInternal & Surface

O=P

∆ ppt

ligandexchange

stearylphosphate*

Co2+ O=P

4

O=P-O-O

-O

Trioctyl Phosphine Oxide TOPO

TOPOCo2+

Co2+Co2+

Co2+

Co2+

Co2+

= Co2+

Co2+Co2+

as made pre-treated TOPO/SP treated% (by mol) dopant (% mol) (% mol)

Db2_107 Co:TiO2 7 ~57 3

Co2+

Co2+TiO2

2.0

1.5

1.0

0.5

0.0

Abs

orba

nce

(a.u

.)

800700600500400300Wavelength (nm)

Absorption SpectroscopyAbsorption Spectroscopy

Co2+:TiO2

TiO2

NanocrystalNanocrystal Size by TEM and XSize by TEM and X--RayRay

5 nm

6050403020

SummarySummary

• Controlling both the type of water available and the hydrolysis rate can force an interaction between both metals and give a suitable in-situ mixed-metal precursor

• Autoclave treatments crystallize in solution

• Surface cleaning of nanocrystals improve speciation homogeneity.

• Other metals can be included: Ni2+, Cr3+ & Mn2+.

-200

-150

-100

-50

0

50

100

mag

netiz

atio

n (a

.u.)

30 28 26 24 22 20 18 16 14energy x103

(cm-1)

1.0

0.8

0.6

0.4

0.2

0

abso

rban

ce (a

.u.) 40

20

0

abs

x10-3

20 18 16 14

energy (103 cm-1)

300 KCo2+:TiO2

Co2+:ZnO

MCD of TOPO treated MCD of TOPO treated CoCo0.030.03 TiTi0.970.97OO22 Site Symmetry Site Symmetry

InvestigationInvestigation

e= 0.2 CoCo0.030.03 TiTi0.970.97OO22

e = 1.2 Co(H2O)62+

0.17 M TiO2

5 K

6 T

20

15

10

5

0

mag

netiz

atio

n (a

.u.)

0.40.30.20.10.0BH/2kT

18 281 cm-1

15 723 cm-1

Interstitial Sites in the Anatase LatticeInterstitial Sites in the Anatase LatticeAnatase with interstitials

4b8d

two interstitial sites in anatase from experimental Cu+/Al3+

doping of anatase

4b4b Interstitial Site in AnataseInterstitial Site in Anatase

2.778 A

1.934 A

•Symmetry of 4b interstitial site is -4m2 or D2d.

•Elongated axial ligands,appears more like 4 coord. Co2+

•Distorted Square planar C2

78.98°

101.9 °

Cu2+ 87 pmTi4+ 75 pmCo2+ 89 pm

Ionic Radii

• evidence from Cu2+ implanted anatase from EPR

I.V. Grebenshikov, et al. Phys. Stat. Sol. b, 197785, (1976).

8d Interstitial Site

66.15 °

113.85 °

2.258 A

94.94°

84.06°

1.849A

Site Symmetry2/m or C2h

Compression of axial ligands and elongationof equitorial ligands. No experimental evidenceobserved

Terms and Terms and OrbitalsOrbitals for Dfor D2d2d geometrygeometry

4F4T1g

4T2g

4A2g

Oh D4h

4E1g

4B2g

D2d

4E

4B701 cm-1

0

4E1g4B1g

4B1g

7423 cm-14B

4E 8226 cm-1

4A 16 776 cm-1

t2g

eg

(zy, xz)

xy

x2-y2

z2

D4h

(zy, xz)xy

eb

x2-y2

z2 ab

D2dOh

highsymmetryoctahedral

low symmetry anatase site

increasing distortion

simulated

Cobalt Speciation XANES / EXAFSCobalt Speciation XANES / EXAFS

1.6

1.2

0.8

0.4

0.0

norm

aliz

ed a

bsor

ptio

n

40200-20E-E0 (eV)

nc

film

Co metal

EXAFS XANES

collaboration with Dr. Steve Heald Argonne National Lab - APS

from analysisCo – O 2.08 C.N. = 5.1 Oxygens

NanocrsytalNanocrsytal DMS DMS Length ScalesLength Scales

DMS nanocrystal

< 10nm

nano

electronically coupledcluster of nanocrystal

meso

continuous film of DMS nanocrystals

micro

Colloids Clusters Films10-400 nm > 400 nm

Co:TiOCo:TiO22 NanocrystalNanocrystal MagnetismMagnetism

5 nm

2.0

1.5

1.0

0.5

0.0

Abs

orba

nce

(a.u

.)

800700600500400300Wavelength (nm)

Co: TiO2TiO2

6

4

2

0

-2

-4m

agne

tizat

ion

(10-3

em

u/g)

-4 -2 0 2 4 Field (kOe)

DMS nanocrystal

< 10nm

nanoscale

single isolated colloids (capped)paramagnetic

Co2+

Co2+TiO2

300 K

Co:TiOCo:TiO22 NanocrystalNanocrystal Crystalline SizesCrystalline Sizes

electronically coupledcluster of nanocrystals

meso scale

Inte

nsity

(a.u

.)

6050403020degrees 2θ

22 nm600 °C 30 min

5 nm

Co:TiOCo:TiO22 Magnetism at the Magnetism at the MesoscaleMesoscale

-8

-4

0

4

8µ

B /Co x10

-3

-1 0 1Field (kOe)

-10

0

10

µ Β/C

o x1

0-3

-5 0 5Field (kOe)

5 nm

22 nm22 nm

5 nm

•Ferromagnetism increasing with annealing

300K 300K

• spin coating/annealing nanocrystal precursor

Co:TiOCo:TiO22 Nanocrystal Nanocrystal –– MicroscaleMicroscale

polycrystallinesapphire substrate

0.3

0.2

0.1

0.0

-0.1

-0.2

-0.3

µ B/C

o2+

-4 -2 0 2 4Field (kOe)

-0.2

-0.1

0.0

0.1

0.2

µ B/C

o2+

-1 0 1Field (kOe)

continuous film of DMS nanocrystals

micro

300 K

300 K

SummarySummaryControlling both the type of water available and the hydrolysis rate can

force an interaction between both metals and give a suitable in-situmixed-metal precursor

• Autoclave treatments crystallize in solution

• Surface cleaning of nanocrystals improve speciation homogeneity.• Magnetism can be “turned on” using varied length scale assemblies

• Thermal treatment may be adding carriers via the creation of defect/vacancies

• Thin film greater degree of ferromagnetism– substrate influence