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J. Phys. Chem. B 2005, 109, 23466-23472
Functionalization of Carbon Nanotubes via Nitrogen Glow Discharge Bishun Khare,* Patrick Wilhite,† Benjamin Tran, Elico Teixeira, Kenneth Fresquez, Delphine Nna Mvondo, Charles Bauschlicher, Jr., and M. Meyyappan Center for Nanotechnology, NASA Ames Research Center, Moffett Field, California 94035 ReceiVed: July 7, 2005; In Final Form: September 1, 2005
We have exposed single-wall carbon nanotubes (SWCNTs) to microwave-generated N2 plasma with the aim to functionalize the nanotubes. The results strongly depend on the distance between the discharge source and the sample, since nitrogen atoms generated can be lost due to recombination. No functionalization was observed when this distance was 7.0 cm. At intermediate distances (2.5 cm), the incorporation of nitrogen and oxygen onto the SWCNT was observed, while, at short distances (1 cm), products containing CtN were also observed.
I. Introduction Single-wall carbon nanotubes (SWCNTs) have interesting electrical and mechanical properties and therefore have been the focus of extensive investigations.1 It has been realized that the range of mechanical and electrical properties of carbon tubes can be extended by functionalization of the sidewalls. Mickelson et al.2 fluorinated SWCNTs and then, using typical organic chemistry techniques, replaced the fluorine with alkanes.3 While this opens some avenues for functionalization, it is not possible to easily form all products of interest. We have shown4-6 that cold plasmas offer an alternative route to functionalization of nanotubes, adding, for example, hydrogen to the SWCNTs, which would be difficult to achieve using other techniques. It is known that materials with nitrogen-bearing functional groups serve many useful applications including polymer lubrication7 and increasing the electrical conductivity of plasma copolymerized films.8 The opposite is true for fluorination of SWCNTs which decreases the electrical conductivity.2 Moreover, the addition of nitrile groups to organic compounds increases the water solubility of the compound.9 Thus, nitrogenfunctionalized SWCNTs may find applications in electronics, composites, and materials chemistry. In a previous study,6 we experimented with an ammonia discharge and the functionalized samples showed characteristic peaks corresponding to CsH and NsH stretches and the Hs NsH bend but not CtN. In this work, we examine the effects of a N2 discharge on functionalization, through characterization of functionalized nanotubes using Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and UV-vis-NIR spectroscopy. The products of functionalization are sensitive to the distance between the nanotube sample location and the discharge source. When the discharge is located close to the nanotubes, products containing CtN are observed. II. Experiment The SWCNTs produced and supplied by Rice University were purified according to a procedure described before.4 The purified nanotubes were suspended in 2-propanol and then deposited on CaF2 disks (25 mm × 2 mm). The samples were allowed to * Corresponding author. E-mail:
[email protected]. † Presently at New York University.
dry under a heat lamp to remove any residual 2-propanol. The IR spectra of the samples resembled SWCNTs with impurities in the aliphatic C-H stretching region (∼2900 cm-1). As described in ref 4 and references therein, hydrocarbons have a high affinity for attaching to nanotube sidewalls. These bands are not due to 2-propanol, since its distinct features are not present in the IR spectra. We have shown that heating the nanotubes under an inert atmosphere removes these C-H stretching features; however, they do reappear over time. Furthermore, monitoring of these band intensities allows for a better assessment of any surface chemistry occurring during the discharge. A microwave discharge scheme similar to the one in ref 6 was used to generate a plasma with N2 as the input gas. The nitrogen gas employed was ultrahigh purity grade 99.999% with impurities of CO2 and CO < 1 ppm, O2 < 2 ppm, THC < 0.5 ppm, and H2O < 3 ppm. In this setup, the sample is not immersed directly in the plasma, as in common plasma processing reactors; instead, as seen in Figure 1, it remains outside the discharge production zone to reduce the detrimental effects associated with ion bombardment. Thus, the exact location of the sample and its distance from the discharge source becomes a factor in determining the extent of functionalization. The source consists of a quartz tube (inner diameter of 10 mm and 1 mm thickness) inserted in a McCarroll cavity operated at 2.45 GHz by an Opthos microwave generator (model MPG4M). The net delivered power was maintained at 100 W. The discharge was found to be most stable in the pressure range 300-550 mTorr and was operated at the lower end of the threshold, since this typically results in higher gas dissociation factors.10-13 The SWCNT samples were placed 7, 2.5, and 1 cm from the stainless steel plug in various experiments. For the two long distances, typical pressure readings at the exit of the sample compartment ranged from 20 to 45 mTorr. The pressure differential was created by using a stainless steel collimating plug with a 1 mm bore down the center and positioned at the end of the discharge tube. An Ultra-Torr fitting, which was connected directly to the discharge, held the steel plug in place while the exit of the tube was connected to a vacuum. The entrance hole of the stainless steel plug was purposely designed off-axis to filter out UV radiation produced within the plasma.4 This enabled only the plasma species to pass through and interact
10.1021/jp0537254 CCC: $30.25 © 2005 American Chemical Society Published on Web 11/15/2005
Functionalization of CNTs via Nitrogen Glow Discharge
Figure 1. Schematic of the experimental setup.
with the sample. Functionalization runs with the N2 plasma were performed for various periods up to 4 h, and in each case, the time history of the process was obtained by running the process for 5, 15, 30, and 45 min. An exposure of 1 h was found to produce adequate intensities of the bands upon functionalization and, thus, is the focus of the discussion here. For the 1 cm distance runs, we altered the dimensions of the inlet and outlet of the stainless steel plug from 1 mm to the largest possible hole that would not allow UV rays to reach the nanotubes (Figure 1). This enabled more plasma species to impact the target before recombination to molecular nitrogen. After increasing the size of the aperture, there was a significant decrease in the time it took for N2 functionalization to occur. The other end of the Cajon fitting was connected to a vacuum. The pressure of the gas leading to the discharge cavity was kept at approximately 250 mTorr, while the exit end was at approximately 1 mTorr. A Thermo-Nicolet Nexus 670 FTIR instrument was used to collect infrared spectra at 4 cm-1 resolution. X-ray photoelectron spectroscopy (XPS) spectra were collected with a PHI Quantum 2000 spectrometer using a monochromatic Al KR (1486.6 eV) source. The integrated sampling depth was 50-100 Å, and the area analyzed was 1400 µm × 300 µm. Scans were performed at 0.8 and 0.1 eV resolutions for the survey and the highresolution spectra, respectively. Various sensitivity factors were taken into account by the spectrometer software. XPS survey scans of the 1 cm plasma exposed SWCNT were carried out using a HP 5950 ESCA spectrometer. The instrument employs
J. Phys. Chem. B, Vol. 109, No. 49, 2005 23467 a monochromatic Al KR source and was operated at a power of 600 W. The integrated sampling depth was 20-40 Å. Nonlinear least squares curve fitting was applied to fit Gaussian/ Lorentzian curves to find peak maxima and to estimate the chemical states of carbon, nitrogen, and oxygen. Raman spectroscopy was performed on the 2.5 cm plasma exposed SWCNT with a Nicolet 670 FTIR instrument with a Raman attachment. The excitation wavelength was 1.064 µm from a Nd:YAG laser with a laser power of 0.37 W. A Nicolet InGaAs detector was used. The number of scans was 128 in 266.4 s with a resolution of 4 cm-1. In addition, a Renishaw Raman spectrometer using a 514 nm argon ion laser was used in the analysis of the 1 cm plasma exposed SWCNT. An 1800 lines/ mm grating was used for the optimized throughput as well as resolution for the excitation. A set of double-notch filters was used to block the laser Rayleigh scattering. The spectral resolution is better than 4 cm-1 in the 4000-100 cm-1 spectral window. Most representative spectra are chosen for presentation here, though the entire sample area was probed at each excitation. III. Results and Discussion A. 7 and 2.5 cm Sample Plasma Distances. Essentially no functionalization was detected at the longest distance studied (7 cm). Recombination of atomic nitrogen is a serious problem14 with increasing distance from the plasma source through substantial contribution from wall recombination and, to a lesser extent, three-body recombination. A Teflon tube or Teflon coating of the quartz tube can reduce the wall recombination probability by 2-3 orders of magnitude. When the distance was shortened to 2.5 cm, products were detected upon exposure to the N2 plasma. The FTIR spectrum of the exposed SWCNTs displays bands characteristic of NsH stretching (3350 cm-1, Figure 2A); amide I, CdN stretching, and/or HsNsH bending (1672 cm-1); and amide II, CdN, and/or CdC (1564 cm-1). The features in the FTIR spectra are very much similar to those taken in a previous study6 that used NH3 as the source gas. Associated with the growth of the new bands is the reduction in the CsH bands (see Figure 2B); therefore, it seems likely that most of the NsH modes arise from N reactions with the hydrogen already attached to the purified SWCNTs. However, we cannot rule out that some of the hydrogen could come from the walls of the vacuum system and, to a lesser extent, from
Figure 2. FTIR results for the 2.5 cm sample distance from the plasma. Part A (3500-2800 cm-1) shows the N-H stretching region of SWCNTs (a) before exposure, (b) after 45 min of N2 plasma exposure, and (c) after 40 min of NH3 plasma exposure from ref 6. Part B (3050-2750 cm-1) shows reduction of aliphatic C-H stretching as a function of plasma exposure time (a) before exposure and (b) after 15, (c) 30, and (d) 45 min of N2 plasma exposure.
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Figure 3. (A) Changes in peaks in the radial breathing mode (a) before and (b) after 1 h of exposure to N2 plasma at a sample distance of 2.5 cm. The changes in the peaks are due to changes in nanotube radii upon functionalization. (B) Red shift in the FT-Raman spectra for the SWCNT G-band (a) before exposure at 1585 cm-1 and (b) after exposure to 1581 cm-1.
impurities in the discharge gas. In our previous study,6 it was also found that, for a ND3 microwave discharge under similar conditions, NsH stretching bands were present and were more apparent than NsD stretching bands. The amine stretching bands produced in this N2 discharge are less intense than those produced under an NH3 discharge. FT-Raman spectral analysis reveals structural changes in the nanotubes by an examination of the radial breathing mode (RBM) region15 below 500 cm-1. Changes in the Raman intensities of the peaks in the RBM region, as seen in Figure 3A, after exposure to nitrogen plasma suggest stresses that accompany functionalization. Specifically, the peaks in the Raman spectrum of the unfunctionalized SWCNTs at 320, 266, 242, 238, 228, 213, and 188 cm-1 (Figure 3A) show significant change after 1 h of plasma exposure, with a complete disappearance of the 320 cm-1 peak, a significant change in the intensity and band position of the peaks at 188 and 228 cm-1, and a weak band at 238 cm-1 to a relatively strong band at 242 cm-1. In addition, the tangential modes between 1100 and 1800 cm-1 (G-band and D-band) also show change (Figure 3B). The G-band peak at 1585 cm-1 exhibits a red shift after plasma exposure to 1581 cm-1, thus providing evidence of functionalization. There is also an increase in the peak at 1926 cm-1 after functionalization, assignable to CdO stretch (not shown here). XPS analysis of SWCNT samples before plasma exposure reveals relative percentage compositions for C, N, and O as
90.1, 0.0, and 5.8%, respectively. The remaining 4.1% is due to CaF2 substrate (1.5%) and Na (0.1%), S (0.2%), and Fe (2.3%) impurities, the latter two being impurities even after SWCNT purification. After 1 h of N2 plasma exposure, the N content increased to 9.6%, while the C and O values were 65.8 and 20.6%, respectively. The oxygen impurity in the samples is due to the fact that XPS was not done in situ and the sample was transferred in air from the microwave chamber to the X-ray photoelectron spectrometer. The relatively higher amount of O in the plasma exposed sample may be due to the larger number of defects following plasma exposure. Figure 4 shows that the maximum for the nitrogen peak (1s) was found to be at 399.8 eV, slightly shifted from the 399.5 eV maximum found in NH3functionalized SWCNTs, and is assignable to amine, amide, and nitriles. The latter is bound to have a minor contribution due to the absence of any discernible signal in FTIR. This subtle difference could be understood by the difference in elemental compositions and bonding. Higher levels of oxidization shift the peak maximum to higher binding energies. The peak can be further deconvoluted into two peaks at 399.0 eV (amine/ nitrile) and 400.0 eV (amide). There is no evidence for di,trioxidized nitrogen groups at higher binding energies. The carbon region of unfunctionalized SWCNTs displays peaks at 284.5, 285.5, and 286.2 eV (Figure 5A). The main peak at 284.5 eV is assigned to C sp2 in the nanotube structure, while a small portion of this peak could also be due to C-H from impurities as seen in the FTIR analysis. The small feature
Functionalization of CNTs via Nitrogen Glow Discharge
Figure 4. XPS N 1s spectrum of functionalized SWCNTs for a 2.5 cm sample distance. Dashed lines show further deconvolution of the peak into two peaks at 399.0 eV (amine/nitride) and 400 eV (amide).
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Figure 6. XPS O 1s spectrum of functionalized SWCNTs for a 2.5 cm sample distance. Dashed lines show further deconvolution showing three peaks at 530.0, 531.6, and 533.1 eV.
TABLE 1: Chemical States of Carbon before and after N2 Discharge sample
Figure 5. XPS C 1s spectrum. (A) unfunctionalized SWCNTs: The main peak is due to C-C bonding, while the less intense ones are probably due to oxygenated groups. (B) Functionalized SWCNTs for a 2.5 cm sample distance.
at 285.5 eV could be C-O or perhaps a peak tailing of C sp3. The remaining peak is likely due to oxygen-containing groups (Table 1). After functionalization, the C-C peak maximum
binding energy (eV)
assignment
control SWCNT
284.5 285.5 286.2
C 1s CsC/CsH C 1s CsO C 1s CdO
functionalized SWCNT for the 2.5 cm distance
284.8 285.7 286.8 287.9 288.9
C 1s CsC/CsH C 1s CsO/CsN/CsC/CsH C 1s CdO C 1s OsCdO C 1s CdC shake-up/CO3
shifts to 284.8 eV (Figure 5B). This is indicative of rehybridization of sp2 carbon to sp3 carbon from functionalization. Further curve fitting revealed peaks at 285.7, 286.8, 287.9, and 288.9 eV. The possible assignments for these peaks are provided in Table 1. Before functionalization, the maximum in the O 1s region is at 532.0 eV (not shown here). At least one more peak can be seen at 529.9 eV which is partially due to trace amounts of iron oxide. After functionalization, the peak maximum shifts to 531.5 eV and was further resolved into three peaks at 530.0, 531.6, and 533.1 eV (Figure 6), indicating the presence of several different bonding structures. In summary, at 2.5 cm, we observe a variety of products that suggest N atoms have inserted into C-H bonds on the tube or reacted with other hydrogen-containing species, either hydrogencontaining impurities in the gas or physisorbed on the tubes to create NHn species. These species lead to the same products that we found in our previous study6 using NH3 as the source gas. B. 1 cm Sample Plasma Distance. When we shorten the distance and increase the flow, there are dramatic changes in the products. The FTIR spectrum of the SWCNTs upon exposure to the N2 plasma (see Figure 7) now displays bands characteristic of CtN (2189 and 2090 cm-1) in addition to the NsH stretching (3350 cm-1); amide I, CdN stretching, and/or HsNsH bending (1672 cm-1); and amide II, CdN, and/or Cd C (1564 cm-1) observed previously. Other proposed forms of nitrogen attachments cannot be confirmed due to the transmission cutoff by the CaF2 substrate. Evidence for nitrogen attachment to SWCNTs appears just after 5 min of exposure to the N2 plasma, as seen by the presence
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Figure 7. (A) FTIR spectrum of (a) the control (SWCNT without exposure to plasma species of the discharge) revealing no features at 3250 cm-1 and subsequent spectra after (b) 5, (c) 15, (d) 30, and (e) 45 min of nitrogen plasma exposure to the SWCNT (for a 1 cm sample distance) show a gradual red shift in band maxima from ∼3280 to 3240 cm-1 and also an increase in the band maxima, assignable to N-H stretching. (B) FTIR spectra of SWCNT (a) before exposure and after (b) 5, (c) 15, (d) 30, and (e) 45 min of exposure to nitrogen plasma for the 1 cm sample distance. An increase in the band intensity at around 2189 and 2090 cm-1 is evident. These two peaks are assignable to CtN bands.
of a broad peak around 3250 cm-1 after functionalization assignable to primary amine (N-H) stretching. After subsequent exposures of 5, 15, 30, and 45 min of nitrogen plasma bombardment, this peak slightly shifts from 3280 to 3240 cm-1 and increases in intensity. There is little change in the intensity after 45 min, suggesting a saturation point for the attachment of amines to the sidewalls. In addition to primary amine peaks, we also observe the presence of two strong peaks at 2189 and 2090 cm-1 (Figure 7B) assignable to cyanides (CtN). The exact characteristics and intermediate reactions that cause it to attach to the nanotube surface cannot be pinpointed; however, nitrogen can react with sidewall defects or an aliphatic group bonded to the sidewall. We suspect that bombarding the SWCNT surfaces with very energetic species such as N2+ helps create defects. Even though the nanotubes are not directly immersed in the plasma, the close proximity (1 cm) and large aperture on the plug may lead N2+ to the nanotubes. Therefore, a longer plasma exposure time may result in more defect sites, thus increasing the amount of cyanides. It is possible to eliminate a few other sources of the CtN structure. It cannot be due to the reaction with sidewall species such as CH3, which otherwise would have happened in all samples regardless of distance from the plasma source as well as even in NH3 based experiments. It is known from Titan tholin production experiments16 that CtN species may form as part of a deposition film. Such a film can occur in the presence of
methane and nitrogen with any form of energy source. Here, methane could be an impurity, though unlikely, or the CsH could play the needed role. The spectral peak for this bond in tholin resides around the same region as in our nitrogen exposed nanotubes ((4 wavenumbers). As a result, it could be possible that our peak is the result of the formation of a tholin-like film on the surface of the nanotubes. However, in the comparison of the spectra of tholin and the nitrogen exposed nanotubes, it is clear that the strongest tholin peaks are not present in our sample, and therefore, this gives us further evidence to conclude that our peak at 2185 cm-1 must be the result of CtN formed in the SWCNTs. The CtN bands exhibit little change in intensity after 3045 min of exposure, relative to the intensity change from 15 to 30 min (Figure 7B), thus supporting the possibility of a saturation point due to a lack of space on nanotube sidewalls, similar to the amine features. The latter peak at 2090 cm-1 appears as a shoulder to the first peak at 2189 cm-1 only after 15 min and longer exposures. The Raman spectra for this case are shown in Figure 8. The tangential modes between 1100 and 1800 cm-1 show changes for both the case of amine aliphatic attachment to nanotube sidewalls and functionalization via Ct N at defect sites on the tubes. The G-band region around 1590 cm-1 shows a broad line shape characteristic of metallic SWCNTs. This region also exhibits noticeable alterations; specifically, there is a slight blue shift in both functionalized cases (two different aperture sizes). There is a noticeable
Functionalization of CNTs via Nitrogen Glow Discharge
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Figure 8. (A) Raman spectra showing changes in the 1 cm sample distance SWCNT G-band around 1590 cm-1 (a) before exposure and (b) after exposure exhibiting the CtN resulting from the modified bigger aperture with a noticible increase in peak intensity at 1583 cm-1. (c) Our previous exposure at 2.5 cm which exhibited an abundance of NsH bonds has no appearance of a 1583 cm-1 peak. (B) Changes in the peaks at 266, 248, 228, 207, 204, and 187 cm-1 due to change in nanotube radii after exposure to N from microwave plasma at a 1 cm sample distance: (b) previous work;6 (c) present work. The results for the control sample are shown in part a.
increase in the peak intensity at 1583 cm-1 when the aperture is made larger, allowing more active species to reach the nanotube film. As was found previously, there is also a slight increase in the peak at 1926 cm-1 after functionalization, assignable to CdO stretching (not shown here). Changes in the Raman intensities of the peaks in the RBM region after exposure to nitrogen plasma suggest stresses that accompany functionalization. Specifically, peaks in the Raman spectrum of our control SWCNTs at 266, 248, 228, 207, 204, and 187 cm-1 (Figure 8B) exhibit radical change in Raman intensities. After half an hour of plasma exposure, intensities were either dramatically reduced or completely disappeared. This provides further evidence that structural changes in the SWCNTs have occurred. Chen et al.15 also refer to a peak at 1730 cm-1, which provides evidence of SWCNTs. This peak is present in our control, yet after attachment of nitrogen in the form of CtN, there is a noticeable decrease in this peak. This could signify that the nanotubes have undergone structural modifications and that they have lost some integrity as singlewalled tubes due to the defects that we suspect to have occurred. In summary, when the SWCNT-plasma distance is very short (1 cm) and the plug hole is enlarged, we find CtN products
along with those found for the longer distances. The requirement that the distance be shortened before CtN is incorporated suggests that some more highly energetic species are involved that are eliminated by recombination at the longer distances. Since it seems unlikely that CtN is present at short distances and not the longer distances, we speculate that some highly reactive species, like N2+, is either reacting on the tube to form the CtN products or reacting with the tube to produce defects that subsequently react with nitrogen-containing species to form the observed CtN products. Since the CtN products are not observed at longer distances, we speculate that the reaction is not occurring at existing defects, but rather at defects created by a highly reactive species at a shorter distance. The UV-vis-NIR spectrum is shown in Figure 9. The absorption edge or band edge is defined as the transition between the strong short-wavelength absorption and the weak longwavelength absorption in the spectrum of a solid, generally a semiconductor. The spectral position of this edge is determined by the energy separation between the valence and conduction bands of the material in question.17 From the complete spectrum of the SWCNTs before functionalization (Figure 9), we do not see any evidence of an absorption edge, indicating that our
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Khare et al. there is a general need to eliminate UV photons from impinging on the nanotubes, since they were found to defunctionalize the nanotubes.4 This necessitates the operation of a remote plasma source wherein the sample lies outside of the plasma production zone. Then, we find that the products depend on the distance between the sample and the plasma. At the shortest distance studied, CtN products are observed. We speculate that their formation is due to reactions of highly reactive species, either directly forming the CtN products or indirectly by first causing a defect that subsequently reacts to form the CtN products. In addition, we find NsH and HNH products as was found previously for NH3 plasmas.6 Acknowledgment. Ben Tran is on an internship from Milpitas High School and will start his freshman year at Stanford University in the Fall of 2005. Elico Teixeira is an undergraduate research intern from Santa Clara University. We acknowledge helpful discussions with Page S. Tuminello.
Figure 9. UV-vis spectra of (a) 1 cm sample distance functionalized SWCNTs and (b) unfunctionalized SWCNTs. The change in transmission in part a compared to part b is due to N atom incorporation and variance in thickness of the SWCNT film on the control sample compared to the functionalized SWCNT sample.
unfunctionalized nanotubes exhibit metallic characteristics; this is supported by the Raman spectra in Figure 8A showing the broadline G-band feature characteristic of metallic nanotubes.18 After functionalization, we do not see any evidence of an absorption edge either, thus suggesting that our SWCNTs have not changed in metallic characteristic to any significant degree. The change in transmission after functionalization can be attributed to the variation in thickness of the control SWCNT film compared to the functionalized SWCNT film. Because of the functionalization which causes a change in the density of the SWCNT, we can infer that the dielectric properties of the nanotubes have changed upon functionalization. Furthermore, a change in density will cause a change in the real (n) and imaginary (k) part of the refractive index (N). Since the real (1) and imaginary (2) parts of the dielectric constant () are related to n and k by the relations 1 ) n2 - k2 and 2 ) 2nk, changes in n and k translate to a change in the dielectric properties of the SWCNTs. IV. Concluding Remarks We have shown that a microwave-generated N2 plasma could be a suitable source for incorporating nitrogen functional groups into SWCNTs via sidewall attachment. The efficiency of this approach depends on the production efficiency of atomic nitrogen (i.e., dissociation efficiency of N2). There is significant literature on this examining various types of plasma sources (dc, rf, microwave, electron cyclotron resonance, etc.), gas pressure, and other parameters. Regardless of the plasma source,
References and Notes (1) Carbon Nanotubes: Science and Applications; Meyyappan, M., Ed.; CRC Press: Boca Raton, FL, 1998. (2) Mickelson, E. T.; Huffman, C. B.; Rinzler, A. G.; Smalley, R. E.; Hauge, R. H.; Margave, J. L. Chem. Phys. Lett. 1998, 296, 188. (3) Boul, P. J.; Liu, J.; Mickelson, E. T.; Huffman, C. B.; Ericson, L. M.; Chaing, I. W.; Smith, K. A.; Colbert, D. T.; Hauge, R. H.; Margave, J. L.; Smalley, R. E. Chem. Phys. Lett. 1999, 310, 367. (4) Khare, B. N.; Meyyappan, M.; Cassell, A. M.; Nguyen, C. V.; Han, J. Nano Lett. 2002, 2, 73. Khare, B. N.; Meyyappan, M.; Kralj, J.; Wilhite, P.; Sisay, M.; Imanaka, H.; Koehne, J.; Bauschlicher, C. W. Appl. Phys. Lett. 2002, 81, 5237. (5) Khare, B. N.; Wilhite, P.; Meyyappan, M. Nanotechnology 2004, 15, 1650. (6) Khare, B. N.; Wilhite, P.; Quinn, R. C.; Chen, B.; Schingler, R. H.; Tran, B.; Imanaka, H.; So, C.; Bauschlicher, C. W.; Meyyappan, M. J. Phys. Chem. B 2004, 108, 8166. (7) Brulet, D.; Chauvel, B. Polymer Lubrication Oil Additives Bearing Nitrogen Groups and their use as Additives for Lubrication. France Patent 4229308, October 21, 1980. (8) Nakano, T.; Koike, S.; Ohki, Y. J. Phys. D.: Appl. Phys. 1990, 23, 711. (9) Roberts, J. D.; Caserio, M. C. Basic Principles of Organic Chemistry; W. A. Benjamin, Inc., New York, Amsterdam, 1965. (10) Fan, Z. Y.; Newman, N. Appl. Phys. Lett. 1998, 73, 456. (11) Vaudo, R. P.; Cook, J. W.; Schetzina, J. F. J. Cryst. Growth 1994, 138, 430. (12) Spence, D.; Steingraber, O. J. ReV. Sci. Instrum. 1988, 59, 246. (13) McCollough, R. W.; Geddes, J.; Croucher, J. A.; Woolsey, J. M.; Higgins, D. P.; Schlapp, M.; Gilbody, H. B. J. Vac. Sci. Technol., A 1996, 14, 152. (14) Wang, W.; Hammond, R. H.; Arnason, S. B.; Bensley, M. R. J. Vac. Sci. Technol., A 1999, 17, 183. (15) Chen, B.; Parker, G., II; Han, J.; Meyyappan, M.; Cassell, A. M.; McClure, T.; Dresselhaus, G.; Dresselhaus, M. S. Chem. Mater. 2002, 14, 1891. (16) Khare, B. N.; Sagan, C.; Arakawa, E. T.; Suits, F.; Callcott, T. A.; Williams, M. W. Icarus 1984, 60, 127-137. (17) Perkampus, H.-H. Encyclopedia of Spectroscopy; VCH: Weinheim, 1995 (ISBN 3-527-29281). (18) Pimenta, M. A.; Marucci, A.; Empedocles, S. A.; Bawendi, M. G.; Hanlon, E. B.; Rao, A. M.; Eklund, P. C.; Smalley, R. E.; Dresselhaus, G.; Dresselhaus, M. S. Phys. ReV. B 1998, 58, 24.
