NT ‘06 , Nagano

Carbon Nanotube and Graphene Chemical Sensors

Peter C. EklundDept’s of Physics and

Materials Science and EngineeringPennsylvania State University

University Park, PA 16802 USA

pce3@psu.edu

NT ‘06 , Nagano

Outline

• SWNT and Nanowire Chemical Sensors– Where are we?– Thermoelectric Sensing of Gases with SWNT Mats

• Physisorption (e.g., C6H2n ring molecules)• Gas Collisions (inert gases)

• On to Graphene– n-graphene layer films (nGLs)– Phonon properties of nGLs– Sensor properties—not yet!

• Summary and Conclusions

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Red (before) Blue (after) PSA

In2O3In2O3

SWNT SWNT

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Static Gas-SWNT Interactions

+

-

van der Waals covalent ionic

weak strong intermediate

Physi-sorption Chemi-sorption

One might presume that only the chemisorbed atoms/molecules will significantly effect the transport of electrons in the tube wall….

current

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Structural Considerations

EB= 0.119 eV�σ = 45 m2/gEB= 0.062 eV�σ = 783 m2/g

EB= 0.089 eV�σ = 22 m2/gEB= 0.049 eV�σ = 483 m2/g

�

pore

surface

groove

channel

•Molecular adsorption at interstitial, groove and pore sites.

•No “bulk atoms”…only surface atoms…electrical transport should be sensitive to gas:SWNT interactions

•It takes time for atoms and molecules to saturate the surface via diffusion and to gain access to internal pores and channels

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c.f., H. Romero, G. U. Sumansekera, G.D. Mahan and P. C. Eklund, Phys. Rev. B., 65 (2002)

V2

{ CN (Constantan)

{ {

Cu CN Cu

HEATTT Δ+ TSWNT spaghetti

V1 V3

1 mm x 1mmx 0.2 mm

S = Seebeck Coefficient = V2/ΔT ; V2 = Open Circuit Voltage

Thermoelectric Power (TEP)

Hot Coldee

e+ -

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Two tubes in parallel (e.g., metallic and semiconducting)

S = (1/σ)[S1σ1 +S2σ2] ~ S1 and σ =[σ1 +σ2]~ σ1

Metallic tube (σ1) dominates: i.e., σ1>> σ2

=> Metallic tubes dominate the bundle transport

S =-π 2kB

2

3eT

d lnσ (E)dE

⎛ ⎝ ⎜

⎞ ⎠ ⎟

E F

Mott relation for a Metal

σ1 , S1

σ2 , S2

metallicsemiconducting

TEP(S) and Conductivity(σ) of a SWNT Bundle

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Effect of additional impurity scattering channel

ρI << ρ0

S =-π 2kB

2

3eT

dlnσ (E)dE

⎛ ⎝ ⎜

⎞ ⎠ ⎟

EF

Mott relation for a Metal

σ(E)=e2v(E)2D(E)τ(E) σ = conductivity

v- free carrier velocity, D-density of states, τ-carrier lifetime

ρ = ρ0 + ρIMatheissen’s rule ⇒

⇒ S = SO +π 2kB

2T3e

ρI

ρ0

⎛

⎝ ⎜ ⎜

⎞

⎠ ⎟ ⎟

1τ I

dτ I

dE−

1τO

dτO

dE

⎛

⎝ ⎜ ⎜

⎞

⎠ ⎟ ⎟

EF

ρI due to gas

c.f., papers by: P.C. Eklund, H. Romero, et al.

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H20

C4H90H

C3H70H

C2H50H

CH30H

Linear => Physisorption

(Linear) Thermopower vs. Extra Resistance(Physisorption)

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Resistive Response to C6H2n Adsorption

*Increase in R related to scattering via π-electron coupling.

*Pyridine decreases the resistivity via creation of additional carriers.

3.0

2.8

2.6

2.4

2.2

Fou

r pro

be re

sist

ance

(Ω)

6543210Time (Hrs)

N

vapors in equilibrium with the liquid at 40 °C

R0

n=3

n=4

n=5

n=6

pyridine

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Effects of Gas:SWNT Collisions

Gas molecule collisions with the tube wall should affect the electron mean free path:

• ?Direct scattering of electrons by atom collisions?

• ?Generate a transient “dent”; non-thermal phonons then scatter electrons?

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TEP Response to Inert Gases(Same sample: P=1atm and T=500 K)

-50

-45

-40

-35

-30

-25

S (µ

V/K

)

24201612840Time (hrs)

Ne

-50

-45

-40

-35

-30

-25

S (µ

V/K

)

24201612840Time (hrs)

Ar

-50

-45

-40

-35

-30

-25

S (µ

V/K

)

24201612840Time (hrs)

Kr

-50

-45

-40

-35

-30

-25

S (µ

V/K

)

24201612840Time (hrs)

Xe

-50

-45

-40

-35

-30

-25

S (µ

V/K

)

24201612840Time (hrs)

He

ΔS

SWNTs: PLV,buckypaper, purified at Rice Univ., annealed at Penn State in vacuum at 1000°C, 8hr

c.f., H. Romero, K. Bolton, A. Rosen and P. C. Eklund, Science (307) 2005

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Inert Gas Collisions: TEP vs ResistivityS vs Δρ is linear, in accord with the Mott Equation and

a new scattering channel (gas collisons)

Slope depends on the mass of the colliding atom

P = 1atmT = 500 K

Same film sample; successive exposure to series of inert gases

c.f., H. Romero, K. Bolton, A. Rosen and P. C. Eklund, Science (307) 2005

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Computed Transient Power Spectrum (Local Vibrational Modes)

Simulations by K. Bolton and A. Rosen, Goteborg University/Chalmers Tech. Univ.

•Local energy spectrum; Derived from motion of C-atom nearest the collision(Ttube= 0 K).

•The amplitude of vibration, not the frequencies, are sensitive to the collideratom mass m

•Radial component of kinetic energy is key parameter

•The tube wall is dented, and then it rings in the low frequency “squash” mode

•Significant vibrational energy remains after 10 ps; 2-acoustic phonon decay

-1500 cm

m=He,Ne,Xe(10,0)

0 cm-1

40 cm-1

Θi = 45°, KE =13kcal/mol

After 5 ps

After 10 ps

c.f., H. Romero, K. Bolton, A. Rosen and P. C. Eklund, Science (307) 2005

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TEP (S) vs Pressure

Saturation => Dent-Dent overlap at ~ 0.2 - 0.5 atm ??

c.f., H. Romero, K. Bolton, A. Rosen and P. C. Eklund, Science (307) 2005

(T = 500 K)

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Why do (S,ρ) saturate with pressure?

• Q = P/(2πmkT)1/2 collisions/area.time, P = pressure

• Overlapping Dents QAτ = 1 collision, Area A~d2

•Use Kinetic theory of Gases; Ask: What collision rate Q causes two dents with lifetime τ and diameter d to overlap?

He Ar Xedent lifetime (τ) ~100 psec ~100 psec ~100 psecdent diameter (d) 1.5 nm 4.3 nm 6.4 nmPsat(calc) 0.97 atm 0.66 atm 0.54 atmPsat(expt) 0.79 atm 0.56 atm 0.53 atm

diameter d

adjacent dents

c.f., H. Romero, K. Bolton, A. Rosen and P. C. Eklund, Science (307) 2005

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Mn Power Laws : Expt and Theory

All slopes in the plot represent M1/3 behavior

D

Mass M

“NanoDent”

T = 500 K P = 1 atm

c.f., H. Romero, K. Bolton, A. Rosen and P. C. Eklund, Science (307) 2005

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Phonon Properties of UltrathinGraphitic Films

SWNT

Graphene

How are the phonon properties related???

nGL

Si : SiO2

nGL=n-layer graphene film

3-layer graphene

film

3-layer graphene

film

Let’s study the phonon properties vs n!

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Motivation

• Exciting transport results in ultrathin graphitic films containing a few atomic layers have been reported– `High carrier mobility; gate-controlled transport;

Quantum Hall effect (Kim et al; de Heer et al)

• We call these carbon systems n-graphene layer films or nGL’s

• Using phonons to probe the system vs number of layers (n)…..how will the system evolve?

nGL

Si : SiO2

nGLnGL

Si : SiO2

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nGL Film Preparation

substrate

n nGL

substrate

n nGL

SiO2: Si

n=2

n=1

n=8

n=19

n=5

HOPG

(top view)

(side view)

Film thickness measured by AFM z-scans

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a) Plane view and b) electron diffraction pattern showing six-fold symmetry. In (a), the nGL has small contrast relative to the carbon TEM grid. Angles θ, θ’ in the figure are 120°, the angle between basal plane vectors in graphite

a) b)a) b)

θ

θ'

TEM image (a) and SAD (b) of nGL

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AFM z-scans of n=3 nGLsubstrate n=3 nGL

Film thicknes via averaging the step heights of ~100 line scans

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nGL thickness h(n) vs assigned n

8

6

4

2

0

Hei

ght (

nm)

20151050Estimated layers (n)

h = C*n + D

C = 0.35 ± 0.01 nmD = 0.33 ± 0.05 nm

h(1)= 0.7 nm

•AFM z-scan used to measure the height relative to substrate

•Least squares fit h vs n:

Slope= 0.35 nm ; slightly larger than c/2=0.335 nm

•h(1)= 0.35 + 0.33 = 0.68 nm extra thickness may reflect inherent difference in attractive AFM tip force

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8

6

4

2

0

Inte

nsity

(A. U

.)

3000250020001500Wave Number ( cm-1)

n=1

n=2

n=3

n=4

n=5

n=8n=19

HOPG

G-band

High Freq. Raman Spectrum of nGLs

2nd order

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G-band vs n

1 6 0 0

Raman Frequency Shift ( cm-1)

n=1n=2n=3n=4n=5n=8n=19

HOPG

~6 cm-1

substrate

n nGL

G-bands are all well fit with a single Lorentzian (Voight analysis)

“G”

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G-Band Frequency vs 1/n

substrate

n nGL

HOPG

ωG ~ 1/n

IIIIIII∞ n=1234510

“G”

5.5 cm-1

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Phonon and Electron Dispersion in Graphene

c.f., C. Mapeli et al., Politecnico di Milano (1998) in A. Ferrari and J. Robertson, Phys Rev B61 (2000)

~1375 cm-1

~1500 cm-1

~710 cm-1

Phonon DOS

phonons

electrons

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Hg line G-band

1 2

Other weak 1st order Scattering

D-band

“D”=Disorder-induced Scattering•Dispersive mode

•Zone edge mode

•Intensity decreases with increasing n

1“D”

0.35

0.30

0.25

0.20

0.15

0.10

0.05

160014001200

n=1

n=2

n=3

n=4

n=5

n=8

n=19

HOPG

Raman Frequency Shift ( cm-1)

Ram

an In

tens

ity (C

ount

s/m

W-S

ec)

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D/G Raman Band Intensity vs 1/n

substrate

n nGL

substrate

n=2

n=3

n=12 nm rms

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8

6

4

2

0

Inte

nsity

(A. U

.)

3000250020001500Wave Number ( cm-1)

n=1

n=2

n=3

n=4

n=5

n=8n=19

HOPG

G-band

High Freq. Raman Spectrum of nGLs

2nd order

NT ‘06 , NaganoRaman Frequency Shift ( cm-1)

Inte

nsity

(A.U

.)2nd Order Raman Spectra of nGLs

280027002600250024502400 33003200

n=1

n=2

n=3

n=4

n=5

n=8

n=19

HOPG

n=1

n=2

n=3

n=4

n=5

n=8

n=19

HOPG

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Summary: Chemical Sensors• Carbon Nanotube and Semiconducting

Nanowire FETs show great promise as real-time analytical tools– There is much to be learned about the sensing

mechanism (charge transfer vs electronic scattering)

• HOWEVER, the SWNT cylindrical shell geometry should be the most sensitive. We find it is even sensitive to:– physisorption (with little or no charge transfer)– even collisions of inert gas atoms with the tube wall

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Summary: Raman Scattering from nGLs

• Raman Scattering useful to characterize the number of layers in an nGL film

• 1st order G-band– Contains one Lorentzian– G-band upshifts with decreasing n: ωG(n) ~ ωGraphite + 7 cm-1(1/n)

• 2nd Order bands – Stronger than G-band for n<5 (unusual behavior!)– Mid-freq band (~2700 cm-1) exhibits shape specific to n– ~2450 cm-1 and ~3250 cm-1 bands not sensitive to n

• D-bands– Intensity decreases exponentially with n– D-scattering may indicate an n-dependent bending of the thinnest

films to conform to the substrate roughness– A weak ~1500 cm-1 D-band is also n-specific; downshifts with

increasing n

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Acknowledgements

Penn State: Dr. Gugang ChenAwnish GuptaProf S. TadigadapaPrasoon Joshi

Prof K. Bolton, Chalmers/GoteborgProf Arne Rosen, Chalmers/GoteborgProf. G.D. Mahan, Penn State Dr. Hugo Romero (U, Penn)Prof. G. Sumanasekera (U. Lou’ville)Dr. UnJeong Kim (Samsung)

SWNT Gas Sensors:n-Graphene Layer Films

Funding/Support: United States National Science Foundation, Penn State Materials Research Institute, CarboLex, Inc.

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Supplementary Slides

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(Non-Linear) Thermopower vs. Extra Resistance(Chemisorption)

-60

-50

-40

-30

-20

-10

0

10S

(μV

/K)

80x10-36040200ρa / ρo

O2

NH3

T=300 KO2: weak electron acceptor

NH3: weak electron donor

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P. G. Collins,…, A. Zettl, Science 287, 1801 (2000)G.U. Sumanasekera,…,P.C. Eklund, Phys. Rev. Lett. 85, 1096 (2000)K. Bradley et al, Phys. Rev. Lett. 85, 4361 (2000) H. Romero,…, P. C. Eklund, Phys. Rev. B., 65, 2054 (2002)

30

20

10

0

-10

S (µ

V/K

)

1086420Time (hrs)

O2 desorption

O2 adsorption

T = 500 K

A

B

C

D

S(μV/K)

25

20

15

10

5

0

-5

-10

-15S

(µV

/K)

500400300200100T (K)

(D) High-T O2-doped(irreversible)

(A) Ambient O2-doped(reversible)

(B) Compensated

(C) Degassed @500 Kin vacuum

S~T

S(μV/K)

15

10

Effects of Oxygen Exposure