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Nanomaterials: improving gassensor performance
John Saffell
Alphasense Ltd.
Technical Director
Paul Midgley
Professor of Materials Science
NANOMATERIALS 2010 University of Cambridge
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We will consider:
Technologies and markets for gas sensing
Nanometrology
Nanomaterials as catalysts
Nanomaterials in optical gas sensing
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Technologies and markets ingas detection
A roadmap, which includes the matrix oftechnologies and markets is availableon:
www.gas-sensor-roadmap.com
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Gas detection has manymarkets
Market segments
Domestic safety
Automotive
Industrial safety
Process control
Military
Emerging markets
Niche
Air quality
Homeland security- Explosives/ terrorism
Asthma, allergies
Medical
Hydrogen: fuel cells
Extreme environments (space, volcanoes, oil)
Breath analysis & capnography
Existing markets
Fire and home safety
Leak detection
Car emissions
PM10, PM2.5
Industrial safety & LEL
Confined space entry
Stack emissions
Process control and analysis
Food processing, transport and storage
Breathalyser / alcohol & drugs
Ammonia
Benzene, BTEX
Outdoor air, Indoor air
Odours (WWT, landfill)
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Many technologies areemployed
Components
Lasers and optics
UV, IR, microplasma sources
Wavelength separation MEMS
Low cost optics, detector arrays
Fibre optics
Micro GC
Micro MS
PID, IMS
QMB, SAW, BAW
Sensor arrays
Microprocessors/ FPGAs/ PICs/ ASIC
Wireless
Technologies
MEMS
Nanomaterials (QDs, CNT, catalysts, nano MO)
Polymers, liquid crystals
Electrochemistry
Separation science
Physical chemistry (enthalpy, speed of sound)
Products
NIR spectrometers
IR single line absorption
IMS
Micro GC/MS
Nanoparticle fluorescence
IR, Visible, THz gas cameras
Ultrasound, thermal conductivity imaging
Electrochem/ optical/polymer/ nano arrays
LIDAR, DOAS
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Nanometrology
Electron microscopy and AFM are regular toolsfor both R&D and quality control
Scanning Electrochemical Microscopy,improved Raman and near field microscopy
are offering new opportunities
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TEM: Daresbury analysis of our Pt/Ru catalyst,identifying oxides and allowing us to determine
the growth pattern
3nm
(010)
[001]
(001)
(011)
55o
(100)
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+12.72%
+4.6
5%
RuO2 Ru
- 4.28%
(010)
(100)
(001) (110)
+12.72%
-4.2
8%
(001)
(100)
(i)
(ii)
(iii)
+4.6
5%
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Zngrain
Pt islanddeposited
fromsolution
RuO2
deposited –columnar
growth
Runanocrystals at oxidesurface
Growthmechanism
time
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Oxygen Map
We also use Energy Filtered TEM toidentify the surface activity of our catalysts
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SEM is routinely used to quantify catalystprimary particle size distribution
2 4 6 8 10 120
5
10
15
20
25
30
35
Part
icle
Counts
Particle Diameter (nm)
Dart 181A
2 4 6 8 10 12 14 16 18 200
2
4
6
8
10
12
14
Part
icle
Counts
Particle diameter (nm)
PtBO2
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Nanoparticles agglomerate, so primary particlesize does not tell the entire story
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Ru black
5nm
100nm
Small 2-6 nm particles can agglomerate tolarge particles
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Nanomaterials as catalysts
• We have been using nanomaterials ascatalysts for decades- they have justbeen rebranded as nanoparticles.
• With better analytical tools, we nowhave better control of our catalysts.
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Nanomaterials for gas detection:many choices
•• CNTsCNTs (MW)(MW)
•• CNTsCNTs + polymer+ polymer
•• CNTS + metal oxidesCNTS + metal oxides
•• CNT + metal catalystsCNT + metal catalysts
•• ZnO nanowiresZnO nanowires
•• SnOSnO22 nanonano powderpowder
•• Tungsten oxidesTungsten oxides
•• IIIIII--V quantum dotsV quantum dots
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Many growth/ deposition methods
• CVD
• PVD
• Nanopipette: QDs, MMOs, polymers
• electropolymerisation (polymers)
• in-situ CNT growth
• Flame ablation
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Molecular structure of [Et2In(OS2CNMenBu)]2
NOAH: DTI funded project to make gas sensors fromquantum dots and nanorods using single component
CVD
(Universities of Manchester & Keele, Alphasense, Teer, Epichem)
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SEM image of InS nanorods
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-In2S3 films grown at 375 °C
TEM shows straight In2S3 nanorods
with average diameter of
ca. 20 nm and ca. 400–500 nm
in length.
High-resolution TEM confirms
crystallinity by indicating well-resolved
(103) lattice planes. The experimental
lattice spacing, 0.66 nm is consistent
with the 0.62 nm separation in bulkcrystals.
Good deposition, but poor gas response
TEM image of InS nanorods
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Flame Spray Pyrolysis
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SnO2 particles generated byflame spray pyrolysis
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SnO2 by flame pyrolysis shows goodresponse and strong temperature
dependence
10 ppm C2H5OH (C). The sensors with Pd/Al2O3 filter (filled symbols) and without filter (open symbols)for both undoped SnO2 (black squares) and Pd-doped SnO2 (grey circles) are measured at 50% r.h. at 25°C.
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Carbons
• Graphite
• CNT (single and multiwalled)
• boron doped diamond
• glassy carbon
• graphene?
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5nm
TEM can also be used to follow a processsuch as ball milling of graphite
2nm
2nm
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Increased ball milling increases theamorphous layer thickness
5nm
5nm
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PECVD Chamber for direct growth ofCNT
• Graphite heater usedto heat substrates
• (Plasma) DC Voltage-630V
• Temperature ofGrowth: 550 – 900oC
Rotary PumpConnected to thebell jar
Gas Inlet for Ammonia,Acetylene and Nitrogen
GraphiteStage heaterconnected
Gas Exposureoutlet for thesamples
Top View Showingsamples on agraphite stage
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Direct Growth of Carbon Nanotubes
• Novel Technique to growCNTs direct on chip
• Microheater heated to growCNTs locally on the desiredarea in 5mins in vacuum at0.2mbar.
• MWCNTs grown locally on thesmall heaters , radius 12um.
• SWCNTs can be grown athigher temperature and thinnercatalyst deposition.
Small heater withCNTs
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CNTs Grown on SOI Membranes
ResistiveElectrodeswith CNT
on SOIMembrane
Depositionfor 15minsusing 2nmFe catalyst
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How the CNTs will work assensors
• Gases like NO2 areelectrophillic so it canremove electrons fromCNTs (For SWCNTs)
• For MWCNTs – chargetransfer mechanism.
• CNT conductanceincreases and thereforethe resistance of the filmdecreases.
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Reported CNT response to NO2
Room temperature Response Time (2ppm) = 30sec, Sensitivity = ~15%F.Udrea et al , IEDM 2007, December
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ZnO nanowires
• Nanowire grew properly in case of resistive sensor withAl metallization(Au plated)
• Resistance 10 k – 300 k
Growth on microhotplate: combining MEMS and nanomaterials
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Nanomaterials in optical gassensing
Quantum dots re-emit light at much longer wavelengths thanexcitaion wavelength- this allows us to shift LED emissions tomuch longer wavelengths (Trackdale)
Controlled nanoparticles on surfaces give repeatable SurfaceEnhanced Resonant Raman Spectroscopy (SERRS)
Nanoparticles can replace metal surfaces as the conductinglayer for surface plasmons (SPR)
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Conclusion
• Improved, lower cost analytical tools (electronmicroscopy and AFM) bring quality control tonanomaterials
• Catalyst are being improved with III-V and carbonbased materials now added to our catalyst choices
• Optics are using the unusual emission andconduction properties of nanomaterials
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Acknowledgements
• Paul O’Brien Manchester Chemistry
• Rod Jones Cambridge Chemistry
• Nicolae Barsan University of Tuebingen Physics
• Bill Milne, Sumita Santra and Florin UdreaCambridge Engineering
• James Covington and Julian GardnerWarwick Engineering
• Paul Midgeley and Cate DucattiCambridge Materials Science and Metallurgy
• Cambridge CMOS Sensors
• Daresbury Laboratory
• Technology Strategy Board (ULoGS project funding)
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Thank you for your attention