Supporting Information for
High Performance Organic Transistors: Percolating
arrays of nanotubes functionalized with an electron
deficient olefin
Mandakini Kanungo1, George G. Malliaras 2, and Graciela B. Blanchet 3a)
1
Xerox Corporation, Webster, NY, 2 Ecole Nationale Supérieure des Mines de Saint
Etienne, France, 3Nanoterra, Cambridge, MA, 02135
a) To whom the correspondence should be addressed: e-mail: gblanchet@nanoterra.com
Reagents and Chemicals: Purified High-pressure CO decomposition (HiPco)
single-walled nanotubes (SWNTs) were obtained from Carbon
Nanotechnologies. TCNE was purchased from Sigma-Aldrich.
Synthesis of SWNT-adduct: The commercially purified HiPCO SWNTs were
dried at 250oC at a pressure of < 1 mbar, for overnight and was then transferred
in to a 10 mL PRL shaker tube and required amount of TCNE in dichlorobenzene
was added to the shaker tube. The tube was closed under nitrogen, chilled in dry
ice for 30 minutes, and then evacuated to remove the N2. The reaction was
conducted in PRL at 215oC for 24 hours. After heating the tube was evacuated
through a cold trap. The carbon nanotube was filtered through 0.2 m PTFE
membranes and washed several times with organic solvents. The functionalized
SWNTs were then dispersed in ODCB at a concentration of 300 mg/L and were
horn sonicated for 10 minutes.
TFT Fabrication: TFTs were fabricated on heavily doped silicon wafers with a
3000 oA wafer thermal oxide. Gold source and drain electrodes were
lithographically patterned. The channel dimensions used were W= 200 m, L=20
m. A few drops of dispersed carbon nanotubes were allowed to settle on the
pattered silicon wafer for 3 minutes and then spun coated at a spin speed of 500,
1000 and 2000 rpm. Electrical properties were measured using a standard
Agilent unit. A voltage of 0.01V to 1V was applied between source and drain
electrodes and the gate voltage were swept from 50V to -100V while measuring
the drain current.
AFM Characterization: AFM height and phase images of the purified and
functionalized SWNTs were obtained in tapping mode in air at resonance
frequencies of 50-75 kHz with oscillating amplitudes of 10-100 nm. The AFM
images were obtained using conventional Si tip (k= 3-6N/m) with a Veeco
Metrology Nanoscope IV (Digital instruments, Santa Barbara, CA)
Separation of the reactants on carbon nanotube sidewalls by TGA and XPS:
The separation of the reactants on the carbon nanotube walls was estimated
from the thermal gravimetric analysis (TGA) X-ray Photoelectron Spectroscopy
(XPS) data. For TGA the samples were purged with nitrogen for 2 hours before
the start of the run. The TGA was carried out in a TGA Q500 v6.6 Build 197
instrument. The sample was heated from room temperature to 1000 oC at 10 oC
under nitrogen. The mass loss below 750 oC was assumed associated to the
reactant and that loss above 750 oC with the unreacted CNT.
Sample: FSWNT-TCNE (II) CNT
Size: 1.9490 mg
Method: ISO120 RT to1000°C@10°C N2
Comment: N2 / Blanchet / Job# 270732
File: C:...\Blanchet, Graciela\788216.ug1
Operator: Ken Roth
Run Date: 17-Oct-2008 18:36
Instrument: TGA Q500 V6.6 Build 197
TGA
120
0.4
11.57% wt. loss RT to 332°C in N2
(0.2255mg)
0.3
Weight (%)
32.25% wt. loss 332°C to 549°C in N2
(0.6285mg)
0.2
13.94% wt. loss 549°C to 996°C in N2
(0.2718mg)
80
Residue:
42.52% at 996°C
(0.8286mg)
0.1
60
[ – – – – ] Deriv. Weight (%/°C)
26.40°C
100.3%
100
0.0
40
0
200
400
600
Temperature (°C)
800
Figure 1: TGA of TCNE functionalized HiPco tubes
-0.1
1000
Universal V4.2E TA Instruments
TGA data show that 90% of the carbon nanotubes have reacted by 600 oC for all
compositions with c>0.007. The decomposition/oxidation appears to occur
readily and indiscriminately. In contrast, only 40% of the SWNT reacted upon
PSEPVE functionalization perhaps reflecting that the small molecular size of
TCNE compared to PSEPVE allows not only more reaction sites on carbon
nanotubes walls thus helping in shortening of the tubes but perhaps also enter
the tubes. We proposed that chain cleavage occurs via the radical mechanism
shown below. In this scheme, the cyclobutyl ring undergoes a reverse
cycloaddition reaction and the tubes get open up at that site creating more
reactant sites and at the same time shortening the tubes.
Scheme 1: Proposed radical mechanism for chain cleavage
Measuring the lengths of some 200 ropes in each of seven AFM picture
associated with different concentrations we are able to correlated average chain
length with mobility as shown in figure 2. The roughly constant vertical shift of
one curve with respect to the other indicates a constant factor and hence a
roughly linear dependence of mobility on chain length over the range of
concentrations investigated
Figure 2: Average chain length vs. mobility as a function of CTCNE/CSWNT.
The TCNE functionalization to the carbon nanotubes was further confirmed by
XPS. For XPS analysis the pristine and functionalized carbon nanotubes were
drop dried on to a clean Si wafer. The experiments were carried out with a
Physical Electronics Quantera Scanning ESCA Microprobe, using a focused (100
um) monochromatic Al X-ray (1486.6 eV) beam operated at 18 kV and 100 W.
The X-ray beam is generated using an electron gun and scanned over ~1400
um x ~200 um to define the analysis area. The electron energy analyzer was
operated in the constant energy mode with pass energy of 55 eV and 0.1 eV step
size between points for high resolution elemental scans (typically 20 eV scan for
each element). . The C1s band is also de-convoluted into three main peaks at
284.8, 286.1 and 287.3, associated to C-C, C-N (sp2) and C=N (sp3) indicating
functionalization of the tubes with TCNE.
x 10 4
cTCNE/cSWNT=0.007
4
8
284.8 eV
284.8 eV
C-C
C-C
7
6
Pos.
284.83
286.14
287.37
289.04
290.68
5
4
Sep.
0.00
1.31
2.54
4.21
5.85
%Area
71.48
15.77
4.51
3.55
3.35
4
cTCNE/cSWNT=0.066
3.5
Pos.
284.86
286.14
287.37
289.04
290.68
3
286.0 – 287.5 eV
C-N
3
x 10
Sep.
0.00
1.28
2.51
4.18
5.82
%Area
49.63
20.15
18.06
6.62
3.49
C-C
SP2
C-N
2
SP3
C=N
2
1
1
0
0
295
290
285
Binding Energy (eV)
280
295
Figure 3: XPS analysis of functionalized SWNTs.
290
285
280