NAS Technical Report NAS-05-002, March 2005

An Investigation of Plasma Chemistry for dc

Plasma Enhanced Chemical Vapor Deposition of

Carbon Nanotubes and Nanofibers

David B. Hash,1 Martin S. Bell,2 Kenneth B. K. Teo,2 Brett A. Cruden,1 William I. Milne,2

and M. Meyyappan†1

1
Center for Nanotechnology, NASA Ames Research Center, Moffett Field, California

94035, USA

2
Department of Engineering, University of Cambridge, Cambridge CB2 1PZ, United

Kingdom

The role of plasma in plasma enhanced chemical vapor deposition of carbon nanotubes

and nanofibers is investigated with both experimental and computational diagnostic

techniques. A residual gas analysis (RGA) of a 12 mbar direct current (dc) discharge with

a C2H2/NH3 gas mixture is conducted near the Ni catalyst surface employed for carbon

nanofiber growth. The results are corroborated with a 1-D dc discharge model that solves

for species densities, ion momentum, and ion, electron, and neutral gas thermal energies.

†
Corresponding author. E-mail: M.Meyyappan@nasa.gov.

-1-
 NAS Technical Report NAS-05-002, March 2005

The effect of varying the plasma power from 0 to 200 W on the gas composition is

studied. The dissociation efficiency of the plasma is demonstrated where over 50 percent

of the feedstock is converted to a mixture of hydrogen, nitrogen, and hydrogen cyanide at

200 W. Finally, the important role that endothermic ion-molecule reactions play in this

conversion is established for the first time. Of these reactions, dissociative proton

abstraction and collision-induced dissociation are of the greatest significance.

1. Introduction
Catalytic decomposition of carbon-bearing gases on metallic nanocluster catalysts in

chemical vapor deposition (CVD) systems for growth of carbon nanotubes and

nanofibers combines the advantages of low-temperature (relative to arc discharge and

laser ablation), low-cost, and commercial-scale production with the ability to pattern

growth through lithographic positioning of transition metal catalysts on substrates. Many

potential applications of nanotubes such as atomic force microscope tips [1],

superhydrophobic surfaces [2], field emission cathodes [3,4], vertical interconnects [5,6],

electron beam lithography [7,8,9,10], synthetic membranes [11,12], intracellular gene

delivery devices [13,14], and nanoelectrode electrochemical probes and biosensors

[15,16,17,18,19] require not only patterned growth but also vertical alignment.

Alignment in thermal CVD processes can be obtained through van der Waals interaction-

sponsored alignment where carbon nanotubes are grown closely together like towers or

through template-assisted growth. However, the application of large electric fields

provides superior alignment [20,21,22,23,24] by inducing dipole moments preferentially

along the axes of carbon nanotubes that act to align the tube in the direction of the field

and combat any randomizing effect of thermal vibrations. Electric field enabled

-2-
 NAS Technical Report NAS-05-002, March 2005

alignment is exploited in plasma enhanced CVD (PECVD) with various configurations

including dc [25,26,27,28,29,30,31], radiofrequency [32,33,34,35], microwave

[36,37,38], inductive [39,40,41,42,43,44], and electron cyclotron resonance [45,46,47].

Lin et al. [48] demonstrated that the alignment mechanism is the same for all systems.

Regardless of whether a system employs a microwave plasma with a self-bias of 10 V

and sheath width on the order of 100 µm or a dc discharge with 600 V applied bias and 2

mm sheath width, the sheath electric fields are on the same order and are requisite for

aligned growth. The plasma itself, in fact, is not required for aligned growth as references

[21] and [22] employed fields through small applied voltages (3-20 V) across very small

electrode spacings (10-100 µm) and thus avoided striking a discharge. This demonstrates

that the only requirement for aligned growth is an electric field and that it is the sheath

electric field of the plasma and not the plasma itself that affects alignment. However,

large-scale electric field aligned nanotube production has yet to be accomplished without

a plasma; and thus, as it is at present unavoidable for large-scale superiorly aligned

growth, it is important to understand the comprehensive role of the plasma. Specifically,

the present work examines the effect of the plasma chemistry on gas composition in dc

PECVD and thus the subsequent precursors to nanotube growth. The investigation relies

on both experimental and computational methods to perform plasma diagnostics. Within

the computational approach, the importance of modeling sheath endothermic ion-

molecule reactions is examined. The experimental approach involves a residual gas

analysis (RGA) study of the plasma and adds to the growing, but still limited work in this

area [49,50,51,52,53].

-3-
 NAS Technical Report NAS-05-002, March 2005

2. CNT Growth
The reactor employed for this study is a simple dc configuration of equal area (10 cm2)

cathode and anode with a 5 cm separation. The feedstock of 54:200 sccm of C2H2/NH3 is

injected through a showerhead, which also acts as the anode. The graphite cathode has an

embedded rigid tungsten wire heater coupled with an electrically isolated thermocouple

to allow independent temperature control of the substrate if necessary.

Si<100> substrates were coated with conductive indium tin oxide (15 nm thick) and Ni

(7 nm) thin films by magnetron sputtering. The substrate was placed on the cathode and a

dc glow discharge was initiated at low power and pressure (20 W, 2.5 mbar) in pure NH3.

The power and pressure were then simultaneously increased to 120 W and 12 mbar

respectively, and a cathode temperature of 550 °C was typically obtained after just

one minute. This catalyst pretreatment procedure transformed the Ni thin film into

nanoclusters of the range between 50 to 100 nm. After the one minute NH3 plasma-

annealing step, C2H2 was introduced into the gas mixture and growth was performed at

the desired plasma power for 15 minutes. All cases reported were performed at a pressure

of 12 mbar, and the desired plasma power in the range of 20 to 200 W was achieved by

varying the applied dc bias from 470 to 650 V. The substrate is heated solely by the

plasma, and its temperature varies between 350 and 715 °C over the power range

investigated though the low range is not suitable for growth.

3. Computational Model
The role of the plasma is investigated with a 1-D radially averaged computational

model [54]. Equations for the conservation of species mass, ion momentum, and ion,

electron, and neutral gas thermal energy are solved axially between the two electrodes.

-4-
 NAS Technical Report NAS-05-002, March 2005

The model includes 21 neutral species (H2, H, CH4, CH3, CH2, CH, C2H4, C2H3, C2H2,

C2H, N2, N, NH3, NH2, NH, NNH, HCN, CN, HC3N, H2CN, H2CNH), seven charged

species ( NH +3 , NH +2 , C 2H +2 , C 2H + , H +2 , H + , e), and 200 reactions. As a boundary condition

for the gas energy equation, a cathode energy balance is incorporated to model ion
€bombardment,
€ € thermal
€ € €
radiation, and solid and gas conduction to predict the cathode

temperature.

The computational model includes endothermic ion-molecule reactions that have not

been investigated in simulations heretofore. Inclusion of these reactions requires the

addition of the ion momentum and energy equations to compute both the directed and

thermal energies of the ions. Endothermic ion-molecule reactions only occur in the sheath

where ions have energies larger than the reaction barrier. Studies of low-pressure

discharges for semiconductor applications, in which sheaths are collisionless, have often

neglected endothermic ion-molecule reactions while including exothermic ones. Peko et

al. [55] advocated for the inclusion of ion-molecule reactions in simulations, as they are

“essential because of the important role that secondary products from ion-molecule

reactions play in the etching and deposition processes.” This is especially true in

nanotube processing where sheaths are collisional because of higher operating pressures

(~ 10 mbar) and ion energies are significant because of the large applied biases (~ 500 V)

required for alignment such that both exothermic and endothermic ion-molecule reactions

are important. Table 1 displays the rates employed for exothermic ion-molecule

reactions, and the endothermic rates for charge transfer, dissociative charge transfer,

dissociative proton abstraction, and collision-induced dissociation are given in Table 2.

Rates for the exothermic reactions are easily found in the modeling literature of planetary

-5-
 NAS Technical Report NAS-05-002, March 2005

atmospheres; however, rates of endothermic ion-molecule reactions for most nanotube

processing gases are not available. As a result, the rates of these reactions for methane

from the work of Peko et al. [56] were used as estimates for acetylene and ammonia,

where the experimentally measured cross-sections reported for these reactions were

integrated over a Maxwellian distribution and then fit to the Arrhenius form.

4. Experimental Diagnostics
Mass spectrometry was performed one centimeter from the cathode using a Hiden EQP

High Energy plasma analyzer, differentially pumped to 1×10-6 mbar. For neutral species

measurements, the acquired mass spectra must be deconvolved, as the resulting

intensities, i, from the RGA are products of the original species cracking patterns. For a

system of n species and m spectra, the following matrix [57] must be solved for [I]

 i1   a11 • • • a1n   I1 
    
i2  =  a21 • • • a2n  I2 
(1)
•  • • • • • •
    
im  am1 • • • amn  In 

where a represents the cracking patterns for each of the n species. The 70 eV cracking
€ patterns are taken from the NIST Chemistry WebBook [58]. To avoid overfitting the

data, n was limited to 17 species, which best fit the data and were consistent with the

modeling results.

5. Results and Discussion

Figures 1 and 2 display the percent decomposition of the feedstock gases, ammonia,

and acetylene, as a function of plasma power. It is defined as the percent change in mole

fraction relative to the plasma off condition

X s∗ − X s Ps∗ P ∗ − Ps P
= (2)
X s∗ Ps∗ P ∗

-6-
€
 NAS Technical Report NAS-05-002, March 2005

where X s is the mole fraction of species s, * denotes the plasma off condition, and Ps is

the partial pressure of species s. Given that the total pressure is maintained constant for

all the cases considered and the intensities from the RGA are proportional to the partial

pressures, Eq. (2) can be rewritten as

X s∗ − X s Is∗ − Is
= ∗ . (3)
X s∗ Is

In the figures, three separate sets of data are displayed: the RGA measurements, the
€ simulation with the full reaction set, and the simulation with a reduced reaction set

excluding the endothermic ion-molecule reactions. For ammonia, the comparison

between the experiment and the full simulation is good between 0 and 100 W but suffers

at high powers. For acetylene, even the full simulation is not able to reproduce the

experimental result where over 80 percent of the feedstock acetylene is decomposed at

the highest plasma power. Possible reasons for the discrepancy may include

approximations in the RGA analysis, rate estimates used for ion-molecule reactions, or

loss of carbon to surface reactions. Of these, the rate estimates used are the most probable

source of error. Because cross section measurements for endothermic ion-molecule

reactions in acetylene and ammonia cannot be found in the literature, the exact same rates

for these reactions were employed for both gases. Thus, it is likely that the methane rates

employed more accurately represent ammonia ion-molecule reaction rates than that of

acetylene, explaining the more accurate comparison to experiment for ammonia than for

acetylene. Despite these possible shortcomings, these plots provide an important insight

into some of the relevant plasma chemistry, namely the importance of incorporating

endothermic ion-molecule reactions into the model. The percent difference between the

simulations with and without these reactions is quite significant, ranging from 50 to 200

-7-
 NAS Technical Report NAS-05-002, March 2005

percent with the full simulations providing a better comparison to the experiment for both

ammonia and acetylene. Figures 3 and 4 display the ion loss rate for each of the

endothermic ion-molecule reactions employed for ammonia and acetylene across the

electrode spacing. The rate axis is plotted logarithmically, dramatically demonstrating

that these reactions occur solely in the 1.5 mm sheath adjacent to the cathode. For both

ammonia and acetylene, the dominant endothermic ion-molecule reaction is dissociative

proton abstraction followed by collision-induced dissociation. At least these two reaction

mechanisms should be included in simulations of dc discharges for nanotube processing

to obtain reliable results.

Figure 5 shows the mole fraction trends with plasma power for the three main plasma

products: H2, N2, and HCN. To compare the experiment with the simulations, the

experimental data are calibrated to the simulation at the 100 W midpoint. The simulation

results displayed here are from the full reaction set employing all ion-molecule reactions.

The trends are reproduced well where hydrogen and hydrogen cyanide increase with

increasing power and nitrogen remains relatively flat. The exception is HCN at lower

powers where the simulation shows a much larger increase in HCN than the experiment.

HCN is formed primarily from the reaction CH + HCN ⇔ N + C2H2 whose equilibrium

constant derived backward reaction rate in the simulation was calculated from a

published [59] estimated forward reaction rate of 1.66 ×10−10 cm3/sec. The region of

discrepancy between 0 and 50 W has a significant gas temperature rise, and it may be that
€
in actuality, the reaction has a temperature dependence that would mitigate the

differences between the experiment and simulation.

-8-
 NAS Technical Report NAS-05-002, March 2005

Figure 6 illustrates the significant role plasma plays in nanotube processing. The

simulated mole fractions of the seven most dominant stable species are shown with the

three most significant radicals over the electrode spacing at the highest plasma power

investigated (200 W). The ammonia and acetylene feedstock gases dissociate near the

cathode in the high-energy sheath region resulting in the highest concentration of

radicals. At this power, the plasma can convert enough of the acetylene into hydrogen

cyanide to make it the most dominant carbon-bearing species at the cathode. Figure 7 is a

similar plot in which the species number densities at the cathode where the substrate is

positioned are plotted versus plasma power. Here, it is clear that the growth sample will

see more HCN than C2H2. The dramatic impact this has on growth has been previously

demonstrated [60], whereby, at the same substrate temperature, higher power conditions

result in much shorter nanofibers relative to lower power growth conditions. The lower

growth rate was attributed to acetylene decomposing more readily on Ni catalyst than

HCN given that the C-N triple bond strength [61] is 748 kJ/mol and the H-CCH bond

strength is only 556 kJ/mol.

6. Conclusions
The role of plasma in dc plasma enhanced chemical vapor deposition of carbon

nanofibers has been investigated both experimentally and computationally. Both

demonstrate the significant role plasma plays in determining the gas-phase species

impinging on the catalysts. The diagnostics show that a 12 mbar, 200 W dc discharge

with a 5 cm electrode gap and 10 cm2 area efficiently dissociates the ammonia and

acetylene feedstock gases by as much as 50-60 percent and 60-80 percent, respectively.

The resulting new carbon-bearing species (hydrogen cyanide, methanimine, and

-9-
 NAS Technical Report NAS-05-002, March 2005

methane) will catalyze at different rates than the original feedstock, thus impacting final

growth rates. Depending on the feedstock gas mixture and the catalyst employed, this

feedstock conversion may or may not be advantageous. In our case where Ni catalyst was

used, the generation of hydrogen cyanide at the expense of acetylene at higher powers

actually impeded the nanotube/fiber growth rate, in contrast to conventional thinking

which assumes higher plasma powers would lead to more efficient/higher growth rates.

The result is that the term plasma enhanced CVD becomes a misnomer where the plasma

chemistry actually results in decreased growth rates. In order to produce optimal growth

conditions with the C2H2/NH3 gas mixture, the plasma power is maintained low enough to

reduce the degree of gas conversion while still high enough to guarantee superior

alignment from the plasma sheath fields.

For the first time, the important role of endothermic ion-molecule reactions has been

demonstrated in plasma-assisted nanotube processing. In many applications, these

reactions can be and have been ignored as they occur only in the sheaths and require

significant pressures and ion energies. The high pressures (~ 10 mbar) and high applied

dc biases (~ 500 V) in nanotube processing however present the perfect condition for

these reactions, and it is demonstrated here that neglecting them may lead to errors in

prediction of gas phase composition on the order of a factor of two.

Acknowledgments
M.S.B. acknowledges support from the Engineering and Physical Sciences Research

Council (EPSRC), UK. K.B.K.T. acknowledges the support of the Royal Academy of

Engineering and Christ’s College, Cambridge. B.A.C is supported by the University

Affiliated Research Center (UARC) at NASA Ames operated by the University of

-10-
 NAS Technical Report NAS-05-002, March 2005

California at Santa Cruz under contract NAS2-031434. Discussions with Alan M. Cassell

and Charles W. Bauschlicher, Jr. are acknowledged.

-11-
 NAS Technical Report NAS-05-002, March 2005

Table 1. Exothermic Ion-Molecule Reactions

Reaction Rate (cm3/sec) Reference

Charge Transfer
H + + NH 3 → NH +3 + H 5.2 ×10−9 62
H +2 + H → H + + H 2 6.4 ×10−10 62
H +2 + C 2H 2 → C 2H +2 + H 2 4.82 ×10−9 62
€ H +2 + NH 3 → NH +3 + H 2 € 5.7 ×10−9 62
€ C 2H + + NH 3 → NH +3 + C€2H 1.6 ×10−9 63
€ NH +2 + NH 3 → NH +3 + NH € 1.8 ×10−9 64
2
€ Proton abstraction €
€ C 2H + + H 2 → C 2H +2 + H€ 1.24 ×10−9 62
€ C H + + CH → C H + + €
2 4 2 2 CH 3 3.74 ×10−10 62
+ +
C 2H + C 2H 4 → C 2H + C 2H 3
2 1.71×10−9 65
€ C 2H + + HCN → C 2H +€CN+
2 5.4 ×10−10 62
€ NH +2 + H 2 → NH +3 + H € 1.95 ×10−10 62
€ NH +2 + CH 4 → NH +3 + CH € 9.2 ×10−10 62
3
€ €
Dissociative Charge Transfer
€ H +2 + C 2H 4 → C 2H +2 + H€
2 + H2 8.82 ×10−10 62
€ Other €
H + + C 2H 2 → C 2H + + H 2 4.3 ×10−9 66
€ H +2 + NH → NH +2 + H € 7.6 ×10−10 66

€ €
€ €

-12-
 NAS Technical Report NAS-05-002, March 2005

Table 2. Endothermic Ion-Molecule Reactions

Reaction Rate (cm3/sec) Reference

Charge Transfer
H + + H 2 → H +2 + H 1.57 ×10−9 T−0.13 exp(−2.9 /T) 67
C 2H +2 + H 2 → H +2 + C 2H 2 4.0 ×10−9 T−0.25 exp(−14.6 /T) 56a
NH +3 + H 2 → H +2 + NH 3 4.0 ×10−9 T−0.25 exp(−14.6 /T) 56a
€ Collision-induced Dissociation €
€ C 2H +2 + C 2H 2 → H + + C 2H +€C 2H 2 1.81×10−9 T−0.17 exp(−4.6 /T) 56a
€ NH +3 + NH 3 → H + + NH 2 + € NH 3 1.81×10−9 T−0.17 exp(−4.6 /T) 56a
C 2H +2 + H 2 → H + + C 2H + H 2 6.8 ×10−9 T−0.2 exp(−9.9 /T) 56a
€ NH +3 + H 2 → H + + NH 2 + H€ 2 6.8 ×10−9 T−0.2 exp(−9.9 /T) 56a
€ Dissociative Proton Abstraction€
€ C 2H +2 + C 2H 2 → C 2H + + H 2€
+ C 2H 7.5 ×10−8 T−0.44 exp(−3.8 /T) 56a
€ + +
NH 3 + NH 3 → NH 2 + H 2 + NH 2€ 7.5 ×10−8 T−0.44 exp(−3.8 /T) 56a
Dissociative Charge Transfer
€ H +2 + H 2 → H + + H + H 2 € 1.97 ×10−9 T−0.19 exp(−5.3/T) 67
€ C 2H +2 + C 2H 2 → C 2H + + H +€C 2H 2 3.7 ×10−9 T−0.3 exp(−21.1/T) 56a
NH +3 + NH 3 → NH +2 + H + NH 3 3.7 ×10−9 T−0.3 exp(−21.1/T) 56a
€ C 2H +2 + H 2 → H + + H + C 2H€2 7.8 ×10−9 T−0.39 exp(−14.8 /T) 56a
€ NH +3 + H 2 → H + + H + NH 3€ 7.8 ×10−9 T−0.39 exp(−14.8 /T) 56a
€ €
€ €
€ €
a
estimate from CH4.

-13-
 NAS Technical Report NAS-05-002, March 2005

Figure 1. Percent ammonia decomposition at one centimeter from the cathode. The

circles are experimental measurements, the squares are the full simulation results

including all ion-molecule reactions, and the triangles are the simulation results excluding

endothermic ion-molecule reactions.

-14-
 NAS Technical Report NAS-05-002, March 2005

Figure 2. Percent acetylene decomposition at one centimeter from the cathode. The

circles are experimental measurements, the squares are the full simulation results

including all ion-molecule reactions, and the triangles are the simulation results excluding

endothermic ion-molecule reactions.

-15-
 NAS Technical Report NAS-05-002, March 2005

Figure 3. Ammonia ion loss rate from endothermic ion-molecule reactions in the cathode

sheath. Dissociative proton abstraction and collision-induced dissociation are the

dominant reactions.

-16-
 NAS Technical Report NAS-05-002, March 2005

Figure 4. Acetylene ion loss rate from endothermic ion-molecule reactions in the cathode

sheath. Dissociative proton abstraction and collision-induced dissociation are the

dominant reactions.

-17-
 NAS Technical Report NAS-05-002, March 2005

Figure 5. Mole fraction trends of main plasma products from simulation and experiment

at one centimeter from cathode.

-18-
 NAS Technical Report NAS-05-002, March 2005

Figure 6. Simulated neutral species in the gas phase at 200 W plasma power plotted

versus the distance between the electrodes. The seven most dominant stable species with

the three most significant radical species are shown here.

-19-
 NAS Technical Report NAS-05-002, March 2005

Figure 7. Simulation of the effect of plasma power on gas phase number densities at the

substrate where carbon nanotube growth occurs.

-20-
 NAS Technical Report NAS-05-002, March 2005

(1) Q. Ye, A. M. Cassell, H. Liu, K.-J. Chao, J. Han, and M. Meyyappan, Nano Lett. 4,
1301 (2004).

(2) K. K. S. Lau, J. Bico, K. B. K. Teo, M. Chhowalla, G. A. J. Amaratunga, W. I. Milne,

G. H. McKinley, and K. K. Gleason, Nano Lett. 3, 1701 (2003).

(3) G. Pirio, P. Legagneux, D. Pribat, K. B. K. Teo, M. Chhowalla, G. A. J. Amaratunga,

and W. I. Milne, Nanotechnol. 13, 1 (2002).

(4) L. Gangloff, E. Minoux, K. B. K. Teo P. Vincent, V. T. Semet, V. T. Binh, M. H.

Yang, I. Y. Y. Bu, R. G. Lacerda, G. Pirio, J. P. Schnell, D. Pribat, D. G. Hasko, G.
A. J. Amaratunga, W. I. Milne, and P. Legagneux, Nano Lett. 4, 1575 (2004).

(5) J. Li, Q. Ye, A. Cassell, H. T. Ng, R. Stevens, J. Han, and M. Meyyappan, Appl.
Phys. Lett. 82, 2491 (2003).

(6) Q. Ngo, D. Petranovic, S. Krishnan, A. M. Cassell, Q. Ye, J. Li, M. Meyyappan, and

C. Y. Yang, IEEE Trans. Nanotechnol. 3, 311 (2004).

(7) M. A. Guillorn, A. V. Melechko, V. I. Merkulov, E. D. Ellis, M. L. Simpson, D. H.

Lowndes, L. R. Baylor, and G. J. Bordonaro, J. Vac. Sci. Technol. B 19, 2598
(2001).

(8) L. R. Baylor, D. H. Lowndes, M. L. Simpson, C. E. Thomas, M. A. Guillorn, V. I.

Merkulov, J. H. Wheaton, E. D. Ellis, D. K. Hensley, and A. V. Melechko, J. Vac.
Sci. Technol. B 20, 2646 (2002).

(9) K. B. K. Teo, M. Chhowalla, G. A. J. Amaratunga, W. I. Milne, P. Legagneux, G.

Pirio, L. Gangloff, D. Pribat, V. Semet, V. T. Binh, W. H. Bruenger, J. Eichholz, H.
Hanssen, D. Friedrich, S. B. Lee, D. G. Hasko, and H. Ahmed, J. Vac. Sci. Technol.
B 21, 693 (2003).

-21-
 NAS Technical Report NAS-05-002, March 2005

(10) L. R. Baylor, W. L. Gardner, X. Yang, R. J. Kasica, M. A. Guillorn, B. Blalock, H.

Cui, D. K. Hensley, S. Islam, D. H. Lowndes, A. V. Melechko, V. I. Merkulov, D. C.
Joy, P. D. Rack, M. L. Simpson, and D. K. Thomas, J. Vac. Sci. Technol. B 22, 3021
(2004).

(11) L. Zhang, A. V. Melechko, V. I. Merkulov, M. A. Guillorn, M. L. Simpson, D. H.

Lowndes, and M. J. Doktycz, Appl. Phys. Lett. 81, 135 (2002).

(12) B. L. Fletcher, E. D. Hullander, A. V. Melechko, T. E. McKnight, K. L. Klein, D. K.

Hensley, J. L. Morrell, M. L. Simpson, and M. J. Doktycz, Nano Lett. 4, 1809
(2004).

(13) T. E. McKnight, A. V. Melechko, G. D. Griffin, M. A. Guillorn, V. I. Merkulov, F.

Serna, D. K. Hensley, M. J. Doktycz, D. H. Lowndes, and M. L. Simpson,
Nanotechnol. 14, 551 (2003).

(14) T. E. McKnight, A. V. Melechko, D. K. Hensley, D. G. J. Mann, G. D. Griffin, and

M. L. Simpson, Nano Lett. 4, 1213 (2004).

(15) M. A. Guillorn, T. E. McKnight, A. Melechko, V. I. Merkulov, P. F. Britt, D. W.

Austin, D. H. Lowndes, and M. L. Simpson, J. Appl. Phys. 91, 3824 (2002).

(16) J. Li, H. T. Ng, A. Cassell, W. Fan, H. Chen, Q. Ye, J. Koehne, J. Han, and M.
Meyyappan, Nano Lett. 3, 597 (2003).

(17) J. Koehne, H. Chen, J. Li, A. M. Cassell, Q. Ye, H. T. Ng, J. Han, and M.

Meyyappan, Nanotechnol. 14, 1239 (2003).

(18) Y. Lin, F. Lu, Y. Tu, and Z. Ren, Nano Lett. 4, 191 (2004).

(19) T. E. McKnight, A. V. Melechko, D. W. Austin, T. Sims, M. A. Guillorn, and M. L.

Simpson, J. Phys. Chem. B 108, 7115 (2004).

(20) Y. Avigal and R. Kalish, Appl. Phys. Lett. 78, 2291 (2001).

-22-
 NAS Technical Report NAS-05-002, March 2005

(21) Y. Zhang, A. Chang, J. Cao, Q. Wang, W. Kim, Y. Li, N. Morris, E. Yenilmez, J.

Kong, and H. Dai, Appl. Phys. Lett. 79, 3155 (2001).

(22) A. Ural, Y. Li, and H. Dai, Appl. Phys. Lett. 81, 3464 (2002).

(23) L. Delzeit, R. Stevens, C. Nguyen, and M. Meyyappan, Int. J. Nanoscience 1, 197

(2002).

(24) E. Joselevich and C. M. Lieber, Nano Lett. 2, 1137 (2002).

(25) Z. F. Ren, Z. P. Huang, J. W. Xu, J. H. Wang, P. Bush, M. P. Siegal, and P. N.

Provencio, Science 282, 1105 (1998).

(26) K. B. K. Teo, M. Chhowalla, G. A. J. Amaratunga, W. I. Milne, D. G. Hasko, G.

Pirio, P. Legagneux, F. Wyczisk, and D. Pribat, Appl. Phys. Lett. 79, 1534 (2001).

(27) M. Chhowalla, K. B. K. Teo, C. Ducati, N. L. Rupesinghe, G. A. J. Amaratunga, A.

C. Ferrari, D. Roy, J. Robertson, and W. I. Milne, J. Appl. Phys. 90, 5308 (2001).

(28) V. I. Merkulov, A. V. Melechko, M. A. Guillorn, M. L. Simpson, D. H. Lowndes, J.

H. Whealton, and R. J. Raridon, Appl. Phys. Lett. 80, 4816 (2002).

(29) Q. Yang, C. Xiao, W. Chen, A. K. Singh, T. Asai, and A. Hirose, Diamond Relat.
Mater. 12, 1482 (2003).

(30) Ch. Täschner, F. Pácal, A. Leonhardt, P. Spatenka, K. Bartsch, A. Graff, and R.

Kaltofen, Surf. Coat. Technol. 174-175, 81 (2003).

(31) J. F. AuBuchon, L.-H. Chen, A. I. Gapin, D.-W. Kim, C. Daraio, and S. Jin, Nano
Lett. 4, 1781 (2004).

(32) T. Ikuno, H. Furuta, T. Yamamoto, S. Takahashi, M. Kamizono, S. Honda, M.

Katayama, T. Hirao, and K. Oura, Surf. Interface Anal. 35, 15 (2003).

(33) T. Kato, G.-H. Jeong, T. Hirata, R. Hatakeyama, K. Tohji, and K. Motomiya, Chem.
Phys. Lett. 381, 422 (2003).

-23-
 NAS Technical Report NAS-05-002, March 2005

(34) T. Kato, G.-H. Jeong, T. Hirata, and R. Hatakeyama, Thin Solid Films 457, 2 (2004).

(35) Y. Li, D. Mann, M. Rolandi, W. Kim, A. Ural, S. Hung, A. Javey, J. Cao, D. Wang,
E. Yenilmez, Q. Wang, J. Gibbons, Y. Nishi, and H. Dai, Nano Lett. 4, 317 (2004).

(36) S. H. Tsai, C. W. Chao, C. L. Lee, and H. C. Shih, Appl. Phys. Lett. 74, 3462
(1999).

(37) C. Bower, W. Zhu, S. Jin, and O. Zhou, Appl. Phys. Lett. 77, 830 (2000).

(38) M. Chen, C.-M. Chen, and C.-F. Chen, Thin Solid Films 420-421, 230 (2002).

(39) L. Delzeit, I. McAninch, B. A. Cruden, D. Hash, B. Chen, J. Han, and M.

Meyyappan, J. Appl. Phys. 91, 6027 (2002).

(40) L. Delzeit, C. V. Nguyen, R. M. Stevens, J. Han, and M. Meyyappan, Nanotechnol.

13, 280 (2002).

(41) K. Matthews, B. A. Cruden, B. Chen, M. Meyyappan, and L. Delzeit, J. Nanosci.

Nanotechnol. 2, 475 (2002).

(42) J. B. O. Caughman, L. R. Baylor, M. A. Guillorn, V. I. Merkulov, D. H. Lowndes,

and L. F. Allard, Appl. Phys. Lett. 83, 1207 (2003).

(43) S. Honda, M. Katayama, K. Lee, T. Ikuno, S. Ohkura, K. Oura, H. Furuta, and T.

Hirao, Jpn. J. Appl. Phys. 42, L441 (2003).

(44) K. Lee, M. Katayama, S. Honda, T. Kuzuoka, T. Miyake, Y. Terao, J. Lee, H. Mori,

T. Hirao, and K. Oura, Jpn. J. Appl. Phys. 42, L804 (2003).

(45) C. M. Hsu, C. H. Lin, H. L. Chang, and C. T. Kuo, Thin Solid Films 420-421, 225
(2002).

(46) Y. S. Woo, D. Y. Jeon, I. T. Han. Y. J. Park, H. J. Kim, J. E. Jung, J. M. Kim, and N.

S. Lee, J. Vac. Sci. Technol. B 21, 1660 (2003).

-24-
 NAS Technical Report NAS-05-002, March 2005

(47) C. H. Lin, S. H. Lee, C. M. Hsu, and C. T. Kuo, Diamond Relat. Mater. 13, 2147
(2004).

(48) C. C. Lin, I. C. Leu, J. H. Yen, and M. H. Hon, Nanotechnol. 15, 176 (2004).

(49) Y. S. Woo, D. S. Jeon, I. T. Han, N. S. Lee, J. E. Jung, and J. M. Kim, Diamond

Relat. Mater. 11, 59 (2002).

(50) B. A. Cruden, A. M. Cassell, Q. Ye, and M. Meyyappan, J. Appl. Phys. 94, 4070
(2003).

(51) I. B. Denysenko, S. Xu, J. D. Long, P. P. Rutkevych, N. A. Azarenkov, and K.

Ostrikov, J. Appl. Phys. 95, 2713 (2004).

(52) M. S. Bell, R. G. Lacerda, K. B. K. Teo, N. L. Rupesinghe, G. A. J. Amaratunga, W.

I. Milne, and M. Chhowalla, Appl. Phys. Lett. 85, 1137 (2004).

(53) B. A. Cruden, A. M. Cassell, D. B. Hash, and M. Meyyappan, J. Appl. Phys. 96,

5284 (2004).

(54) D. Hash, D. Bose, T. R. Govindan, and M. Meyyappan, J. Appl. Phys. 93, 6284
(2003).

(55) B. L. Peko, R. L. Champion, M. V. V. S. Rao, and J. K. Olthoff, J. Appl. Phys. 92,

1657 (2002).

(56) B. L. Peko, I. V. Dyakov, and R. L. Champion, J. Chem. Phys. 109, 5269 (1998).

(57) J. F. O’Hanlon, A User’s Guide to Vacuum Technology (Wiley, New York, 1980), p.
105-119.

(58) NIST Chemistry WebBook, http://webbook.nist.gov

(59) A. J. Dean, R. K. Hanson, and C. T. Bowman, J. Phys. Chem. 95, 3180 (1991).

-25-
 NAS Technical Report NAS-05-002, March 2005

(60) K. B. K. Teo, D. B. Hash, R. G. Lacerda, N. L. Rupesinghe, M. S. Bell, S. H. Dalal,

D. Bose, T. R. Govindan, B. A. Cruden, M. Chhowalla, G. A. J. Amaratunga, M.
Meyyappan, and W. I. Milne, Nano Lett. 4, 921 (2004).

(61) D. R. Lide (Ed.), CRC Handbook of Chemistry and Physics, 81st Edition (CRC Press,
Boca Raton, 2000).

(62) V. G. Anicich, J. Phys. Chem. Ref. Data 22, 1469 (1993).

(63) L. Operti, R. Rabezzana, and G. A. Vaglio, Int. J. Mass Spectrom. 228, 403 (2003).

(64) L. Operti, R. Rabezzana, F. Turco, and G. A. Vaglio, Int. J. Mass Spectrom. 232,
139 (2004).

(65) V. G. Anicich and M. J. McEwan, Planet. Space Sci. 45, 897 (1997).

(66) C. N. Keller, T. E. Cravens, and L. Gan, J. Geophys. Res. 97, 12117 (1992).

(67) A. V. Phelps, J. Phys. Chem. Ref. Data 19, 653 (1990).

-26-