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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