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Rigoni Federica 1° year Phd student federicarigoni@gmail

UNIVERSITY OF MILAN – CATHOLIC UNIVERSITY OF BRESCIA. Sub- ppm ammonia detection in urban environments with carbon nanotubes gas sensors: possible strategies to enhance the sensitivity. Rigoni Federica 1° year Phd student federicarigoni@gmail.com. Carbon nanotubes.

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Rigoni Federica 1° year Phd student federicarigoni@gmail

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  1. UNIVERSITY OF MILAN – CATHOLIC UNIVERSITY OF BRESCIA Sub-ppm ammonia detection in urban environments with carbon nanotubes gas sensors:possible strategies to enhance the sensitivity. Rigoni Federica 1° yearPhdstudent federicarigoni@gmail.com

  2. Carbonnanotubes • Severalallotropicformofcarbon, depending on itshybridization (diamond, graphite, graphene, fullerene, carbonnanotube …) • Manyscientificpapers start citing “Carbonnanotubes, discoveredby Iijima in 1991 …” • Iijimaproduced a newallotropicform ofcarbon (thathecalledmicrotubules ofgraphiticcarbon), usinganarc-discharge evaporationmethodsimilartothat usedfor fullerene (C60) synthesis. Tetrahedral (3D) Trigonal (2D) Linear (1D) sp3 sp2 sp d = 3 nm S. Iijima, Nature354 (1991) 56

  3. What are carbonnanotubes? Roll-up GRAPHENE SHEET CARBON NANOTUBE C hybridization sp2

  4. Single-wallcarbonnanotubeSWNTdiameter 1-3 nm Multi-wallcarbonnanotube MWNT diameter up to 100 nm diameter ≈ nm length ≈ µm 1D crystal Chiralindexes (n,m) (17,0) zig-zag (10,10) armchair (12,8) chiral Different chiralities: different characteristics Ifn-m = multiple of 3 metallic tube otherwise semiconductive tube

  5. Electronic propertiesof SWNT Single wallcarbonnanotubehasdiameter ≈ nm and length ≈ µm, We can consideritas a one-dimensionalcrystal. Density OfStates in a 1D crystal Van Hovesingularities KATAURA PLOT The KATAURA PLOT relatesthe energy of the band gaps in a carbon nanotube and its diameter (in the first-order tight binding approximation). Katauraetal.SyntheticMetals103 (1999) 2555

  6. Carbonnanotubesas gas sensors • CNTs are appealing systems for extremely sensitive gas sensors for at least two reasons: • their one-dimensional nature makes them very sensitive to tiny external perturbations • huge surface-to-volume ratio NO2: OXIDIZING MOLECULE BASIC IDEA: The interactionresulting in a charge transfer between the gas molecule and the carbonnanotubecauses a variation in the electrical conductance (or resistance) of the tube, detectable with an electronic system. NH3: REDUCING MOLECULE Kong et al. Science, 287 (2000) 622

  7. Whymonitoringammonia gas? ppm (parts per million) Hazardoussubstances, explosive, … Environmentalmonitoring ppb (parts per billion) NH3 In urbanenvironment: lessthan 50 ppb NH3isoneof the mainprecursorsofsecondary fine particulate (PM10, PM2.5) Our goal: toenhance the sensitivityofcarbonnanotubesbased gas sensors in ordertodetectsub-ppmconcentrationsof NH3. Ammoniaconcentrationsoverone week in Milan (data source: ARPA Lombardia)

  8. Chemiresistor gas sensor SWNT dispersed in a solutionof water, NaOH, SodiumLaurylSulfate InterdigitatedPtelectrodes Electricalcircuit SWNT bridgesbetweenelectrodes Alumina (ceramic)substrate Methodsofpreparation Drop-casting method Dielectrophoresis method 1 μl 1 μl Alternate Currentappliedduring the deposition (V = 5 V ; f = 1 MHz)

  9. Strategiestoenhance the sensitivityof a SWNT basedchemiresistor • Sonicationof the sample (in ultrasound bath) to reduce the film thickness thinner the film on the substrate, betteris the charge transfer from the gas moleculeto the electricalcontacts. • Dielectrophoresismethodtoalign the SWNT a methodtobetterdistribute the SWNT on the substrateistoapplyan alternate currentbetween the electrodes, during the deposition. In this way SWNTstendstobealigned • Functionalization • Otherarchitectures (e.g. chem-FET) Moscatello et al. MRS, 1057 (2008)

  10. Response: variationof the resistance SENSITIVITY: sub- ppm

  11. Dielectrophoresismethodtoalign the CNT Drop-castingmethod Dielectrophoresismethod 1 μl 1 μl SEM images

  12. (a) ,(b) SWNT on ceramic ID substrate

  13. In literature… There are manyworkson carbonnanotubesasammonia gas sensors, butveryfewofthemreport the detection ofconcentrationsbelow the ppm level. FunctionalizationwithPolyaniline (PANI, a conductivepolymer) Functionalizationwith metal nanoparticles High temperature Penzaet al. Sens. And Act. B, 135(2008) 289 Zhanget al. Electroanalysis, 18 (2006) 1153

  14. Future steps • Functionalization • Differentdeviceconcepts, e.g. chemicalFieldEffect Transistor (chem-FET) CNTs Drain Source S D SiO2 The gateallowstochange the voltage (gatevoltageVg). GATE: p-doped Si

  15. ChemicalFieldEffect Transistor (FET) Vgate = 0 Vgate > 0 more electrons Vgate < 0 more holes K. Uchida et al., Phys. Rev. B 79, 85402 (2009)

  16. Thanksfor the attention!QUESTIONS?

  17. ChemicalFieldEffect Transistor (FET)

  18. ChemicalFieldEffect Transistor (FET) SWNTs S D GATE: p-doped Si Vgate < 0 Vgate = 0 Vgate> 0

  19. Experimental set-up Commercial sensor Based on metal oxides Temperature sensor Chem FET Chemiresistor: SWNT on interdigitatedelectrodes Humiditysensor

  20. Electricalcircuit • Chemiresistor • Chem-FET

  21. Raman

  22. Ramanspectrumof SWNT Ramanspectrumgivesusmany information about the vibrationalmodesofcarbonnanotubes. • Principalpeaks: • RBM: RadialBreathing Mode • (150 - 350 cm¯¹) • D-band: Disorderinduced band • (1350 cm¯¹) • G-band: tangential (derived from the graphite like in-plane) mode • (1560 – 1600 cm¯¹) • G’-band: overtoneofD-band G-band Intensity RBM G’-band D-band Ramanshift (cm¯¹) R. Graupner J. RamanSpectrosc. 38, 673 (2007)

  23. Metallic vs SemiconductiveSWNTs RamanspectraofSWNTs in bundlesusing different excitation energy (2.54, 2.41 and 1.92 eV). The metallic or semiconductingcharacter of the tubes is definitely confirmed by the line-shapeof the TM (G-band). Lorentianprofile RBM G-band semicond. Lorentianprofile semicond. S M Breit-Wigner-Fano profile S metallic S L. Alvarez et al.Chem. Phys. Lett. 316, 186 (2000)

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