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Conference on Potential Benefits of Nanotechnology

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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

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Outline

2

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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Conference on Potential Benefits of Nanotechnology

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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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Conference on Potential Benefits of Nanotechnology

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Chemoresistive gas sensors

4

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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 NO2

concentration: 50 ppb)

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Conference on Potential Benefits of Nanotechnology

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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

11

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

12

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Science, 291 (2001) 1947-1949.

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Why metal oxide nanowires

14

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

15

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

In2O3

Crystal 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

18

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

19

FET based

devices

Bundle geometry Single wire geometry

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Gas sensing performances

(conductometric – bundle geometry)

20

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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

21

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)

<d>= 0.11 m

d (m)

fre

que

ncy

thin wires - seeded substrate

0

5

10

15

20

25

30

a)

<d>= 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

<d>= 0.56 m

Thinner wires enhanced sensing response

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Conference on Potential Benefits of Nanotechnology

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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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Conference on Potential Benefits of Nanotechnology

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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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Conference on Potential Benefits of Nanotechnology

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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)

30

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Single Metal Oxide NW for gas sensing

31

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)

35

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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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Conference on Potential Benefits of Nanotechnology

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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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Conference on Potential Benefits of Nanotechnology

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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