Carbon Nanotube and. Graphene Chemical Sensors. Peter C. Eklund. Dept's of
Physics and. Materials Science and Engineering. Pennsylvania State University.
Carbon Nanotube and Graphene Chemical Sensors Peter C. Eklund Dept’s of Physics and Materials Science and Engineering Pennsylvania State University University Park, PA 16802 USA
[email protected]
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 NT ‘06 , Nagano
Red (before) Blue (after) PSA In2O3
SWNT
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In2O3
SWNT
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Static Gas-SWNT Interactions van der Waals
covalent
ionic
+
-
weak
Physi-sorption
strong
current
intermediate
Chemi-sorption
One might presume that only the chemisorbed atoms/molecules will significantly effect the transport of electrons in the tube wall…. NT ‘06 , Nagano
Structural Considerations surface channel pore groove
EB= 0.119 eV σ = 45 m2/g EB= 0.062 eV σ = 783 m2/g
EB= 0.089 eV σ = 22 m2/g EB= 0.049 eV σ = 483 m2/g
•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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Thermoelectric Power (TEP) T + ΔT
HEAT
T 1 mm x 1mm x 0.2 mm
SWNT spaghetti
Cu
{
V1
CN
Cu
CN (Constantan)
{
{
V3
V2
S = Seebeck Coefficient = V2/ΔT ; V2 = Open Circuit Voltage Hot
+
e
e e
-
Cold
c.f., H. Romero, G. U. Sumansekera, G.D. Mahan and P. C. Eklund, Phys. Rev. B., 65 (2002)
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TEP(S) and Conductivity(σ) of a SWNT Bundle σ 1 , S1 σ 2 , S2
metallic semiconducting
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 2 ⎛ dln σ (E) ⎞ -π 2 k B S= T⎜ ⎟ ⎝ dE ⎠ E F 3e
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Mott relation for a Metal
Effect of additional impurity scattering channel 2 ⎛ dln σ (E) ⎞ -π 2 k B S= T⎜ ⎟ ⎝ dE ⎠ E F 3e
σ(E)= e2v(E) 2D(E)τ (E)
Mott relation for a Metal σ = conductivity
v- free carrier velocity, D-density of states, τ-carrier lifetime Matheissen’s rule ⇒ ⇒
ρ = ρ0 + ρI
ρI due to gas
2 π 2 k B T ⎛ ρ I ⎞⎛ 1 dτ I 1 dτ O ⎞ ⎜⎜ ⎟⎟⎜⎜ ⎟⎟ ρI Physisorption CH30H
C2H50H
C3H70H C4H90H
H20
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Resistive Response to C6H2n Adsorption
R0
Four probe resistance (Ω)
3.0
vapors in equilibrium with the liquid at 40 °C
n=3
2.8
n=4 n=5
2.6
n=6
2.4 pyridine N
2.2
0
1
2
3 4 Time (Hrs)
5
6
*Increase in R related to scattering via π-electron coupling. *Pyridine decreases the resistivity via creation of additional carriers. NT ‘06 , Nagano
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) He
-40
S (µV/K)
-35
ΔS
-40 -45
-50
-50 4
8
12 16 Time (hrs)
20
24
-35 -40 -45 -50
0
4
-25
8
12 16 Time (hrs)
20
0
24
4
8
12 16 Time (hrs)
20
24
-25
Kr
-30 -35 -40
SWNTs: PLV,buckypaper, purified at Rice Univ., annealed at Penn State in vacuum at 1000°C, 8hr
Xe
-30
S (µV/K)
S (µV/K)
Ar
-30
-35
-45
0
Ne
-30
S (µV/K)
-30
S (µV/K)
-25
-25
-25
-35 -40 -45
-45
-50
-50 0
4
8
12 16 Time (hrs)
20
24
0
4
8
12 16 Time (hrs)
20
24
c.f., Romero, K. Bolton, A. Rosen and P. C. Eklund, Science (307) 2005 NTH.‘06 , Nagano
Inert Gas Collisions: TEP vs Resistivity S vs Δρ is linear, in accord with the Mott Equation and a new scattering channel (gas collisons)
P = 1atm T = 500 K Same film sample; successive exposure to series of inert gases
Slope depends on the mass of the colliding atom c.f., Romero, K. Bolton, A. Rosen and P. C. Eklund, Science (307) 2005 NTH.‘06 , Nagano
Computed Transient Power Spectrum (Local Vibrational Modes) Θi = 45°, KE =13kcal/mol
m=He,Ne,Xe
After 5 ps
(10,0)
•The amplitude of vibration, not the frequencies, are sensitive to the collider atom mass m •Radial component of kinetic energy is key parameter
40 cm-1
•The tube wall is dented, and then it rings in the low frequency “squash” mode
After 10 ps
0 cm-1
•Local energy spectrum; Derived from motion of C-atom nearest the collision (Ttube= 0 K).
500 cm-1
•Significant vibrational energy remains after 10 ps; 2-acoustic phonon decay
Simulations by K. Bolton and A. Rosen, Goteborg University/Chalmers Tech. Univ. c.f., Romero, K. Bolton, A. Rosen and P. C. Eklund, Science (307) 2005 NTH.‘06 , Nagano
TEP (S) vs Pressure (T = 500 K)
Saturation => Dent-Dent overlap at ~ 0.2 - 0.5 atm ?? c.f., Romero, K. Bolton, A. Rosen and P. C. Eklund, Science (307) 2005 NTH.‘06 , Nagano
Why do (S,ρ) saturate with pressure? •Use Kinetic theory of Gases; Ask: What collision rate Q causes two dents with lifetime τ and diameter d to overlap? • Q = P/(2πmkT)1/2
collisions/area.time,
• Overlapping Dents He dent lifetime (τ) ~100 psec dent diameter (d) 1.5 nm Psat(calc) 0.97 atm Psat(expt) 0.79 atm
P = pressure
QAτ = 1 collision, Area A~d2 Ar ~100 psec 4.3 nm 0.66 atm 0.56 atm
Xe ~100 psec 6.4 nm 0.54 atm 0.53 atm
c.f., Romero, K. Bolton, A. Rosen and P. C. Eklund, Science (307) 2005 NTH.‘06 , Nagano
diameter d
adjacent dents
Mn Power Laws : Expt and Theory T = 500 K P = 1 atm
Mass M D
“NanoDent”
All slopes in the plot represent M1/3 behavior c.f., Romero, K. Bolton, A. Rosen and P. C. Eklund, Science (307) 2005 NTH.‘06 , Nagano
Phonon Properties of Ultrathin Graphitic Films SWNT
3-layer graphene film
nGL Graphene Si : SiO2
How are the phonon properties related???
nGL=n-layer graphene film
Let’s study the phonon properties vs n! NT ‘06 , Nagano
nGL
Motivation
Si : SiO2
• 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? NT ‘06 , Nagano
nGL Film Preparation
HOPG n=5
n=2 n=8
n=1
n=19
n
nGL substrate
SiO2: Si
(side view)
(top view)
Film thickness measured by AFM z-scans NT ‘06 , Nagano
TEM image (a) and SAD (b) of nGL a)
b) θ'
θ
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 NT ‘06 , Nagano
AFM z-scans of n=3 nGL substrate
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
Height (nm)
8
h = C*n + D
•AFM z-scan used to measure the height relative to substrate
C = 0.35 ± 0.01 nm D = 0.33 ± 0.05 nm
6
•Least squares fit h vs n:
4
Slope= 0.35 nm ; slightly larger than c/2=0.335 nm
2
•h(1)= 0.35 + 0.33 = 0.68 nm extra thickness may reflect inherent difference in attractive AFM tip force
h(1)= 0.7 nm
0 0 NT ‘06 , Nagano
5 10 15 Estimated layers (n)
20
High Freq. Raman Spectrum of nGLs 2nd order
G-band
Intensity (A. U.)
8 6
HOPG n=19 n=8
4
n=5 n=4
2
n=3 n=2
0 1500 NT ‘06 , Nagano
n=1
2000
2500 -1 Wave Number ( cm )
3000
G-band vs n n
nGL
substrate
HOPG
n=19 n=8 n=5 n=4 n=3 n=2 n=1
“G”
~6 cm-1 1 6 0 0
Raman NT ‘06 , Nagano
Frequency Shift ( cm-1)
G-bands are all well fit with a single Lorentzian (Voight analysis)
G-Band Frequency vs 1/n ∞
10 5 4 3 I I I I I
2 I
n=1 I
nGL
n
substrate
ωG ~ 1/n 5.5 cm-1
“G” HOPG
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Phonon and Electron Dispersion in Graphene phonons Phonon DOS ~1500 cm-1 ~1375 cm-1 electrons
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~710 cm-1
c.f., C. Mapeli et al., Politecnico di Milano (1998) in A. Ferrari and J. Robertson, Phys Rev B61 (2000)
Hg line
Raman Intensity (Counts/mW-Sec)
0.35 0.30 0.25
G-band
D-band
HOPG n=19
n=8
“D”=Disorder-induced Scattering
n=5
•Dispersive mode
0.20
•Zone edge mode
n=4
0.15
n=3
0.10
n=2
0.05
Other weak 1st order Scattering
n=1
1200
•Intensity decreases with increasing n
1
2 “D”
1400
1600
NT ‘06 , Nagano Raman Frequency Shift ( cm-1)
D/G Raman Band Intensity vs 1/n n
nGL substrate
2 nm rms
n=1 substrate n=2
n=3
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High Freq. Raman Spectrum of nGLs 2nd order
G-band
Intensity (A. U.)
8 6
HOPG n=19 n=8
4
n=5 n=4
2
n=3 n=2
0 1500 NT ‘06 , Nagano
n=1
2000
2500 -1 Wave Number ( cm )
3000
2nd Order Raman Spectra of nGLs HOPG
HOPG
Intensity (A.U.)
n=19
n=19
n=8
n=8
n=5
n=5
n=4
n=4
n=3
n=3
n=2
n=2
n=1
2400 NT ‘06 , Nagano
2450
2500
2600
2700
n=1 2800
Raman Frequency Shift ( cm-1)
3200
3300
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