crystalline nanowires of semiconducting metal oxides as a ...gas/flavour sensors: wide spectrum of...
TRANSCRIPT
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Conference on Potential Benefits of Nanotechnology
15-17 July 2009, OECD Conference Centre, Paris1
Crystalline nanowires of semiconducting metal
oxides as a new generation of gas sensors
Alberto Vomiero
SENSOR Laboratory , CNR – INFM, Brescia Universityvia Valotti 9, 25133 Brescia ITALY
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Conference on Potential Benefits of Nanotechnology
15-17 July 2009, OECD Conference Centre, Paris
Outline
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Introduction to chemoresistive gas sensors
Quasi one-dimensional oxidic structures
Preparation – Structure & Morphology
Gas sensing performances (conductometric)
Nanowire bundle assembly
Single wire
Single wire array
Gas sensing performances (optical)
Conclusions and outlooks
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Research lines @ SENSOR
Gas/flavour sensors:wide spectrum of bare and mixed metal
oxides like SnO2, In2O3, TiO2, Ga2O3,
SnO2-In2O3 and TiO2-WO3
New activities on nanostructured systems
with reduced dimensionality like
Semiconducting Q1D nanowires
Biosensors
Electronic NoseAn array of gas sensors is the core of the
EOS Electronic Olfactory System or
Electronic Nose.
Learning from data is accomplished with
linear techniques and neural networks.
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Chemoresistive gas sensors
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Solid-State chemical sensors
5
Sensitivity
Selectivity
Stability
Power Consumption
Price
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Formation of a space-charged
region in the grain and a
barrier at the grain boundary
E
EbqVS
conductance G=G0e-qVs/kT
Electrical conduction in ionic materials
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Conductometric reaction based device
Conductivity changes due
to surface reactions with
target analyte
Metal oxide
semiconductor sensors
based on reversible redox
processes of analyte gas
components.
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Conference on Potential Benefits of Nanotechnology
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Sensing characterization
Sub-ppm NO2 detection
RISE * = 160 sec
FALL ** = 10 min
Working T = 100 °C
RH = 30%
Rgas / Rair (100 ppb) = 18
Suitable for outdoor monitoring
applications (threshold NO2concentration: 50 ppb)
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Substrate
2x2 mm2 alumina substrate
Pt heater on the back side
Operating T in the range RT to 500°C
Pt interdigitated contacts sputtered
over the sensing layer
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Gas sensor device
The electronic device is realized bonding the suspended alumina substrate on a TO - 5 or TO- 39
microelectronic case.
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Polycrystalline thick- & thin-film sensors
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Ferroni et al. JVST B (2002)
Grain coalescence
Phase transitions
Guidi et al. Phys. Rev B (2005)
Degradation of sensing performances:
Sensitivity
Stability
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Quasi one-dimensional oxidic structures
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Science, 291 (2001) 1947-1949.
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Why metal oxide nanowires
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Fabrication of increasingly smaller devices & integrated
circuits
Simple bottom-up fabrication methodology
Improved performances & architectures
Sensitivity (enhanced surfave/volume ratio)
Stability (single crystal arrangement)
Selectivity (nanoscopic electronic nose)
Low power (single wire or self standing bundle)
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Experimental set-up
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flow
• The precursor powder is decomposed at high T
• The gas flow is directed to the substrate
• Condensation over the substrate occours at 400-900 °C.
• The substrate can be preliminary seeded/catalyzed
heating elements
Argon inlet
Oxide powders substrates
To pump
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Classical growth of bulk crystals usually leads to
the formation of equi-axed grains with
conventional crystalline habit:
SnO2 crystals – SEM imaging – 3 kev
SnO2 Cassiterite
Crystal system: tetragonal (rutile)
Crystal habit: pyramidic, prismatic
In2O3Crystal system: cubic
Crystal habit: cubic
ZnO
Crystal system: hexagonal (wurzite)
Crystal habit: tapering six-sided pyramids and platy hexagonal crystals
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Quasi uni-dimensional nanostructures
Control of crystal size and shape is needed
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Single crystal arrangement
of oxide nanowires
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HREM atomically-sharp termination of lateral sides
Electron diffractionIa-3 body-centred cubic In2O3 (SG 206)
[100]Growthdirection
HREMCrystalline – lateral sides parallel to growth direction
TEM - BFBend contours
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Conductometric
sensors
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FET based
devices
Bundle geometry Single wire geometry
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Gas sensing performances
(conductometric – bundle geometry)
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Very small lateral sizeSensitivity is not limited by diffusion
Long-term stability for the nanostructurePossibility to reduce the dimension of a sensing device to the nanoscale
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Application of nanowires
to the field of gas-sensing
Effect of particle diameter on
the response to H2 and CO
[N. Yamazoe et al, Catal. Surv. Asia 7 2003]
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0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.40
5
10
15
20
25
30
a)
= 0.11 m
d (m)
fre
que
ncy
thin wires - seeded substrate
0
5
10
15
20
25
30
a)
= 0.11 m
co
un
ts
thin wires - seeded substrate
b)
0,0 0,2 0,4 0,6 0,8 1,0 1,2 1,40
5
10
15
20
25
30
fre
qu
en
cy
d (m)
thick wires - unseeded substrate
= 0.56 m
Thinner wires enhanced sensing response
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Integration of nanowire bundle
Schematic of lift-off based on SiO2 sacrificial layerMOSFET configuration
n-type SnO2
Modulated emission of secondary electrons
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Selective patterning / Electric contacts
Nanowires
Clean substrate
Nanowires
Electric contacts
Ohmic behavior
Main contribution to
electric conduction due
to the nanowires
Negligible contribution
of the contacts
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Ozone detection
SnO2 nanowires
VS
SnO2 thin films
Tin oxide thin film grown via
rheotaxial growth and
thermal oxidation (RGTO)
G. Sberveglieri, Sens. Actuators B (1995)
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DMMP detection(Dymethyl methyl phosphonate simulant for Sarin nerve agents)
28
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Very last achievements
29
Thin film Nanowire bundle
Day 1 Day 46 Day 1 Day 46
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Gas sensing performances
(conductometric - single wire geometry)
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Single Metal Oxide NW for gas sensing
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Gas in
VGS
+ -to backgate contact
VDS+ -
A
Space charge region
VGS
+ -to backgate contact
VDS+ -
A
Space charge region
O-
CO
CO2
e-O
-O-
Hernández-Ramírez, F. et al.
Fabrication and electrical characterization of circuits based on individual tin oxide nanowires
Nanotechnology 17, 5577-5583 (2006).
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Positioning of the nanowire between two Al contacts
Manipulation of single-nanowire
Manipulation / Positioning / integration of single nanowire
is still a major concern for effective single-wire devices
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Gas sensing performances
(Optical)
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Photoluminescence setup
36
• Light Source:
• He-Cd laser. CW @ 326 nm
• UV LED AlGaN/GaN @ 330 nm
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ZnO nanowires at RT: towards detection limit
Gas carrier: humid air
Relative variation
(Areaair-AreaNO2)/AreaNO2
0.1 ppm NO2 (attention level)
detected with relative variation 2.7%
0.2 ppm NO2 (alarm level) detected
with relative variation 3.6%
38
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Conclusions
39
Oxide nanowires:
Easy & cheap synthetic routes
Possibility of integration in nano-devices
State of the art of conductometric NW-based sensors:
Bundle geometry
Single wire geometry
NW-based sensors benefits:
High sensitivity
Enhanced long term stability
Self heating in single wire device
Optical gas sensing (NW-based) explored, towards a RT sensor
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Acknowledgements
Prof. G. Sberveglieri (Director)
Prof. M. Artoni
Prof. G. Faglia
Dr. C. Baratto
Dr. E. Comini
Dr. I. Concina (CNR-INFM)
Dr. M. Falasconi
Dr. M. Ferroni
Dr. V. Galstyan
Dr. I. Kholmanov
Dr. M. Pardo (CNR- INFM)
Dr. A. Ponzoni
Dr. R. Nodari (CNR- INFM, Administrative staff)
Mr. Poli (technical staff)
A. Braga (Ph.D Student)
G. Jimenez (Ph.D Student)
S. Todros (Ph.D Student)
SENSOR Lab Staff
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