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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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slide1

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

carbon nanotubes
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

what are carbon nanotubes
What are carbonnanotubes?

Roll-up

GRAPHENE SHEET

CARBON NANOTUBE

C hybridization sp2

slide4

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

electronic properties of swnt
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

carbon nanotubes as gas sensors
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

why monitoring ammonia gas
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)

chemiresistor gas sensor
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)

strategies to enhance the sensitivity of a swnt based chemiresistor
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)

dielectrophoresis method to align the cnt
Dielectrophoresismethodtoalign the CNT

Drop-castingmethod

Dielectrophoresismethod

1 μl

1 μl

SEM

images

in literature
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

future steps
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

chemical field effect transistor fet
ChemicalFieldEffect Transistor (FET)

Vgate = 0

Vgate > 0

more electrons

Vgate < 0

more holes

K. Uchida et al., Phys. Rev. B 79, 85402 (2009)

chemical field effect transistor fet2
ChemicalFieldEffect Transistor (FET)

SWNTs

S

D

GATE: p-doped Si

Vgate < 0

Vgate = 0

Vgate> 0

experimental set up
Experimental set-up

Commercial sensor

Based on metal oxides

Temperature sensor

Chem FET

Chemiresistor:

SWNT on interdigitatedelectrodes

Humiditysensor

electrical circuit
Electricalcircuit
  • Chemiresistor
  • Chem-FET
raman spectrum of swnt
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)

metallic vs semiconductive swnts
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)