IrO 2

Conductive IrO 2 Nanocrystals
Department of Electronic Engineering,
National Taiwan University of Science and
Technology
黃鶯聲
黃鶯聲
Ying-Sheng Huang
E-mail: ysh@mail.ntust.edu.tw

ACKNOWLEDGMENTS
NSC93,94,95-2120-M011-001
Fred H Pollak, Physics Dept., Brooklyn College of
NYCU
Yun Chi, Dept. of Chemistry, NTHU
Dah-Shyang Tsai, Dept. of Chemical Eng., NTUST
Reui-San Chen, Alexandru Korotcov, Dept. of
Electronic Eng., NTUST
….

Outline
Introduction and background
Growth and characterization of 1D nanosized
IrO 2 crystals
Size effect of 1D nanocrystals studied by
Micro-Raman spectroscopy
Field emission: IrO 2 nanorods
Gas sensing and electrocatalysis applications
Summary

IrO 2 and RuO 2 Single Crystals: The
bigger the better, 1982
IrO 2
RuO 2

Nanosized IrO 2 and RuO 2 :
The smaller the better, 2007
IrO 2 nanotubes
RuO 2 nanorods

Our previous work in single crystal growth
shows the strong 1D growth behavior of IrO 2
Sizes:
2∼30 μm
Length:
0.1∼30 mm

Rod-like IrO 2 crystals reveal clear
prismatic facets

Introduction
IrO 2 belong to the family of conductive transition metal oxides
with rutile-type structure.
The material possesses an interesting variety of electrical
properties
Corrosion-resistant conductive electrode material
Fundamental properties of the conductive oxides, including RuO 2, IrO 2 and OsO 2
Crystal structure
Lattice constant
Bulk resistivity
(μΩ-cm) at 300 K
RuO2
Tetragonal rutile
a = 4.499Å
c = 3.107 Å
20-40
IrO2
Tetragonal rutile
a = 4.498 Å
c = 3.15 4Å
32-50
OsO2
Tetragonal rutile
a = 4.50 Å
c = 3.18 Å
60

Applications of conductive oxides
IrO 2
Optical switching layers in electrochromic
devices
PH-sensing materials
Electrocatalysis for water oxidation
Electrode materials in ferroelectric capacitors
for nonvolatile memories
Emitter materials of field emission cathode
arrays used in vacuum microelectronic
devices and field emission flat panel displays

Advantages of nanosized IrO 2
Low Electrical Resistivity
High Thermal and Chemical Stability
Enhance the material properties by
controlling the microstructure

Growth and characterization
of IrO 2 nanocrystals
Growth techniques:
1. Cold-wall and vertical flow MOCVD
2. RF reactive magnetron sputtering
Characterization:
Morphology→ FESEM
Structure and orientation→ XRD, TEM, SAD
Composition→ XPS, EDS
Microstructure→ Micro-Raman
Possible applications:
Field emitters, Sensors, Electrocatalysis

Schematic diagram of MOCVD system
Precursors:
(MeCp)(COD)Ir
Conditions
Growth
Temp.
( o C)
Growth
Pressure
(Torr)
O2 flow rate
(sccm)
IrO2
350
10-50
100

Schematic diagram of home-made HV RF
magnetron sputtering system
Target
Substrate [Al 2O 3 - sapphire (SA)]
RF power
Background pressure
Sputtering pressure
Substrate temperature
O 2 / Ar flows
1- inch Ir (99.95%)
(001); (100); (012); (110)
65 W
3×10 -5 mBar
~ 6.5×10 -2 mBar
300-400°C
2.5 / 5.0 sccm

Our CVD apparatus

Morphology of IrO 2 nanotubes grown on
rhombohedral LiTaO 3(012) substrate

Morphology of IrO 2 nanorods grown on
rhombohedral LiTaO 3(012) substrate

Morphology of IrO 2 nanorods grown on
rhombohedral LiTaO 3(012) substrate
Size: 50-100 nm

XRD patterns show the growth of IrO 2 on LiTaO 3 (012)
having single orientation of (101)
Intensity
2000
1500
1000
500
0
LTO(012)
IrO 2 (101)
20 25 30 35 40 45 50 55 60 65
2θ

Corss-sectional TEM image and SAD patterns
indicate the epitaxially growth of
IrO 2 nanotubes on LiTaO 3(012) substrate
IrO 2[001]
35 o
IrO 2[101]

IrO 2 nanotubes on various oxide substrates (I)
The growth with vertical alignment can be achieved using these oxide substrates
On YSZ(110) On LiNbO3 (100) On Sapphire(100)

Single variant
alignment
Six variants
alignment
IrO 2 nanotubes on various oxide substrates (II)
On LiTaO 3 (012) On Sapphire(110)
On YSZ(111) On YSZ(100)
Double
variants
alignment
Remark: YSZ is
yttrium stabilized ZrO 2
Eight variants
alignment

IrO 2 nanotubes on YSZ(110) substrate

IrO 2 nanotubes on YSZ(110) substrate

IrO 2 nanotubes on YSZ(110) substrate
Intensity
10000
5000
0
IrO 2 (110)
IrO 2 (101)
YSZ(110)
IrO 2 (211)
IrO 2 (002)
20 25 30 35 40 45 50 55 60 65
2θ

IrO 2 nanotubes on LiNbO 3 (100) substrate

IrO 2 nanotubes on LiNbO 3 (100) substrate

IrO 2 nanotubes on LiNbO 3 (100) substrate
Intensity
12000
10000
8000
6000
4000
2000
0
IrO 2 (002)
LNO(300)
20 25 30 35 40 45 50 55 60 65
2θ

IrO 2 nanotubes on sapphire(100) substrate

IrO 2 nanotubes on sapphire(100) substrate

IrO 2 nanotubes on sapphire(100) substrate

The TEM images and SAD pattern reveal
the 1D IrO 2 crystals with tubular morphology
and have long axes toward [001] direction
[001]

IrO 2 nanotubes on sapphire(100) substrate

IrO 2 nanotubes on sapphire(100) substrate
Intensity
35000
30000
25000
20000
15000
10000
5000
0
IrO 2 (002)
20 25 30 35 40 45 50 55 60 65
2θ

IrO 2 nanotubes on LiTaO 3 (012) substrate
Single
variant
alignment
Intensity
2000
1500
1000
500
0
LTO(012)
IrO 2 (101)
20 25 30 35 40 45 50 55 60 65
2θ

IrO 2 nanotubes on sapphire(110) substrate
Two variants
alignment

IrO 2 nanotubes on sapphire(110) substrate
Two variants
alignment

IrO 2 nanotubes on sapphire(110) substrate
Two variants
alignment
Intensity
8000
6000
4000
2000
0
IrO 2 (101)
SA(110)
20 25 30 35 40 45 50 55 60 65
2θ

IrO 2 nanotubes on YSZ(111) substrate
Six variants
alignment

IrO 2 nanotubes on YSZ(111) substrate
Six variants
alignment

Six variants
alignment
IrO 2 nanotubes on YSZ(111) substrate

IrO 2 nanotubes on YSZ(111) substrate
Six variants
alignment
Intensity
30000
20000
10000
0
YSZ(111)
IrO 2 (101)
20 25 30 35 40 45 50 55 60 65
2θ

IrO 2 nanotubes on YSZ(100) substrate
Eight variants
alignment

IrO 2 nanotubes on YSZ(100) substrate
Eight variants
alignment
Intensity
100000
50000
0
IrO 2 (101)+YSZ(100)
20 25 30 35 40 45 50 55 60 65
2θ

The FESEM images of the vertically aligned IrO 2 nanorods
grown by RFMS on (a) SA(100), (b) LiNbO 3 (100) substrates
and their (c) X-ray diffraction patterns.

IrO 2 (001) on SA (100)
Heteroepitaxy of IrO 2
(001) and SA (100)
produced directional
mismatch of :
- 5.46 % along IrO 2
[100];
+ 3.69 % along IrO 2
[010]

The schematic plots of the lattice relationships between
IrO 2 and sapphire(100) and LiNbO 3(100) substrates:

The FESEM images of the tilted IrO 2 nanocrystals grown by
RFMS on (a) SA(012), (b) SA(110) substrates and their (c)
X-ray diffraction patterns.

IrO 2 (101) on SA (012)
Heteroepitaxy of IrO 2
(101) and SA (012)
produced directional
mismatch of :
-5.46 % along
IrO 2 [010];
+7.02 % along
IrO2 [10 1]

IrO 2 (101) on SA (110)
Heteroepitaxy of IrO 2 (101)
and SA (110) produced
directional mismatch of :
+ 3.69 % // - 4.36% along
IrO2 [010];
+ 2.23 % along IrO2 [ 101]

The FESEM images of the mosaic IrO 2 nanocrystals grown
by RFMS on (a) SA(001) substrates and their (b) X-ray
diffraction patterns.

The top and 30° perspective view FESEM micrographs
and the corresponding schematic plots for (a)−(c) the
nearly triangular nanorods and (d)−(f) the wedge-like
nanorods of IrO 2.

The top and 30° perspective view FESEM micrographs and
the corresponding schematic plots for (a)−(c) the
incomplete nanotubes and (d)−(f) the scrolled nanotubes
of IrO 2.

The top and 30° perspective view FESEM micrographs and
the corresponding schematic plots for (a)−(c) the square
nanotubes, (d)−(f) the intermediate 1D nanocrystals and
(g)−(i) the square nanorods of IrO 2 .

The 30° perspective view FESEM micrographs and the
corresponding schematic plots for (a) and (b) the mixture
comprised of continuous grains and partial short rods, and
(c) and (d) the thin film of IrO 2.

Variation of the surface area being covered, the number
density (inset), and the average size (inset) of IrO 2
nuclei with growth time in the initial growth stage. The
growth temperature is 450 °C.

Arranged IrO 2 nuclei on a SA(012) surface at a growth
temperature of 450 °C and growth times of (a) 30 s and (b)
60 s.

IrO 2 nanorods on SA(012) selectively
grown at 450°C and patterned by the
photolithographic method
(a) a stripe pattern
(b) a corner of square
patch
(c) a border of
populated nanorods
(d) a border of less
populated nanorods

IrO 2 nanorods on SA(100) selectively
grown at 450°C and patterned by the
photolithographic method
(a) a pattern of four
square nongrowth
patches
(b) a corner of square
patch
(c) a corner line of
nanorods
(d) a border line of
nanorods

Nanosized
Single Crystal
XPS of IrO 2

Symmetry for the Raman-active
modes
IrO 2 structure and q=0 displacements, viewed along
the c – axis, for three Raman-active modes.

α y y
Relative Raman intensities for
the E g, B 2g and A 1g modes
Polarization
configuration
α x 'x'
α x 'y'
α y' y'
α z z
α z y
α y y
α x x
α x y
E g
e 2
½ e 2
0
0
e 2
0
0
0
0
Phonon mode
B 2g
(101) face
0
½ d 2
0
(100) face
0
0
0
(001) face
0
d 2
0
A 1g
¼ (a + b) 2
0
a 2
b 2
0
a 2
a 2
0
a 2
x' = 1
2(10
1)
y' = (010)
z' = 1 2 (001)
x = (100)
y = (010)
z = (001)

Spatial correlation model
The phonons in nanometric-sized systems can be confined in space by
crystallite boundaries or surface disorders. That cause an uncertainty in
the wave vector of the phonons and results in downshift and broadening
of the Raman features.
I(
ω)
C
( 0,
3
d q C(
0,
q)
∫ 2
[ ω − ω(
q)]
+ ( Γ / 2)
≅ 2
q
1
, q
2
)
2
≅
e
−q
2
1
L
2
1
/ 16π
2
2
( q) = A + { A − B[
1 cos( πq
ω −
2
2
/ 16π
iq 2 L2
⎞
⎟
32π
⎠
Richter H et al 1981 Solid State Commun. V39 p625; Campbell I H and Fauchet P M 1986 Solid State Commun. V58 p739
e
−q
2
2
)]}
L
2 2
I(ω) - intensity of the firstorder
Raman spectrum
2
1 − erf
⎛
⎜
⎝
1/
2
- one dimensional linear chain model
ω (q) - phonon dispersion curve, Γ - FWHM of the intrinsic Raman line
shape, C (0, q) - the Fourier coefficient of the phonon confinement function
for column shape, L 1 and L 2 are the diameter and length of nanocrystals.
2

Raman spectra show RuO 2 and IrO 2 NCs exhibit apparent
peak red-shift and broadening, which are attributed to the
phonon confinement effect and residual stress

E g mode Raman scattering results of IrO 2
1-D nanostructures – an example of fitting
We will focus our SC analysis
on the E g mode. In order to
have a good agreement
between SC model results
and experimental data we
have to add an additional red
shift in the SC model. We
have tentatively assigned this
shift to the residual stress
effect.
Using our proposed MSC
model, which includes the
effect of the residual stress,
we are able to correlate the
measured red-shift of the
Raman active modes as due
to nanometric size and
residual stress effect.

Sample
IrO 2 (101) on SA (012)
IrO 2 (101) on SA (012)
IrO 2 single crystal
E g (cm -1 )
551.6
561.0
FWHM (cm -1 )
31
12
Δω size (cm -1 )
6.4
diameter L 1 = 40 ± 5 nm; length L 2 ~ 400 nm
L 1 = 39 nm
-
Δω stress (cm -1 )
3
-

Sample
IrO 2 (101) on SA (110)
IrO 2 (101) on SA (110)
IrO 2 single crystal
E g (cm -1 )
551.9
561.0
FWHM (cm -1 )
30
12
Δω size (cm -1 )
6.1
diameter L 1 = 40 ± 5 nm; length L 2 ~ 400 nm
L 1 = 40 nm
-
Δω stress (cm -1 )
3
-

Sample
IrO 2 (100) on SA (001)
IrO 2 (100) on SA (001)
IrO 2 single crystal
E g (cm -1 )
552.6
561.0
FWHM (cm -1 )
33
12
Δω size (cm -1 )
5.4
-
Δω stress (cm -1 )
thicknesses of the NWs L 1 ~ 20 - 45 nm; height L 2 ~ 200 nm; longwise ~ 100 - 200 nm
L 1 = 44 nm
3
-

Sample
IrO 2 (001) on SA (100)
IrO 2 (100) on SA (001)
IrO 2 single crystal
E g (cm -1 )
552.7
561.0
FWHM (cm -1 )
36
12
diameter L 1 = 40 ± 5 nm; length L 2 ~ 400 nm
L 1 = 45 nm
Δω size (cm -1 )
5.3
-
Δω stress (cm -1 )
3
-

Field emission properties of IrO 2 nanorods with
self-assembled sharp-tips

Morphology of the IrO 2 nanorods grown on
titanium(Ti)-coated-Si(100) substrate

The tip size (r) is
around 2-4 nm
Aspect ratio (α)
= h/d = 2.1±0.1
TEM images of IrO 2 nanorods

Field emission J-E curve of IrO 2 nanorods (I)
For IrO 2 nanorods
Turn-on field
(E to ) at 5.6 V/μm
The best values of
carbon nanotube
E to = 0.6 ~ 2.8 V/μm,
N-doped diamond film
E to ~ 1.5 V/μm
amorphous carbon film
E to = 6.0 V/μm

Field emission J-E curve of IrO 2 nanorods (II)
For IrO 2 nanorods,
the lowest field
to drive the emission
(E thr ) at 0.7 V/μm
The best values of
carbon nanotube
E thr = 0.6 ~ 0.8 V/μm,
N-doped diamond film
E thr ~ 0.5 V/μm
amorphous carbon film
E thr = 3 ~ 5 V/μm

Field emission long-term stability of IrO 2 nanorods
From the slope
of FN plot,
the calculated field
enhancement factor
β = 40000 ± 8000,
which is compatible
with the carbon
nanotubes
β = 1000 ~ 50000
Long-term test shows the durable and
robust ability of field emission of IrO 2
nanorods

Typical gas response curves for the IrO2 sensor. (a)
Upon exposure to 1000 ppm propionic acid and (b)
1000 ppm n-hexylamine. IrO 2 was deposited at Ts =
350 ºC, Tpr = 95 ºC.

Summary(I)
Using the techniques of MOCVD and RF reactive
sputtering, 1D nanosized conductive oxide
crystals have been successfully grown on various
substrates with different orientations.
A strong substrate effect on the alignment of the
IrO 2 nanocrystalline growth has been observed.
XPS spectra show that Ir in IrO 2 nanorods also
exist in a higher oxidation state.

Summary(II)
The Raman scattering results reveal three Ramanactive
modes, identified as E g, B 2g and A 1g. The
intensity of certain modes depends on orientation of
NCs and follows the selection rules. This indicates a
possibility to determine the preferable growth
direction of NCs under Raman investigations.
The Raman peaks red shifts and the asymmetric
broadening describe the trends due to phonon
confined effect of the nanoscaled dimensions and
residual stress.

Summary(III)
The field emission results demonstrate the IrO 2
nanorods are emitters of potential.
Area-selective growth of IrO 2 nanorods have been
carried out on silica patterned SA(012) and SA(100)
substrates using MOCVD.

Future Work
Basic physical properties study on the nanosized
IrO 2/RuO 2 , TiO 2/RuO 2
Interface properties between substrates and
IrO 2/RuO 2, TiO 2/RuO 2 NCs
Possible applications will be studied with the
controlled growth of the IrO 2/RuO 2 , TiO 2/RuO 2
nanocrystals
Thanks very much for your attention!

Anatase-TiO 2 /SA(100)

Rutile-TiO 2 /SA(100)

RuO 2 on TiO 2