NANO LETTERS
Transparent Boron-Doped Carbon Nanotube Films
2008 Vol. 8, No. 9 2613-2619
X. M. Liu,† H. E. Romero,† H. R. Gutierrez,† K. Adu,† and P. C. Eklund*,†,‡ Department of Physics and Department of Materials Science, The PennsylVania, State UniVersity, UniVersity Park, PennsylVania 16802 Received November 14, 2007; Revised Manuscript Received May 13, 2008
ABSTRACT We report results of studies on the sheet resistance and optical transmission of thin films of boron-doped single-walled carbon nanotubes (SWNTs). Boron doping was carried out by exposure of SWNTs to B2O3 and NH3 at 900 °C and 1-3 atom % boron was found in the SWNT bundles via electron energy loss spectroscopy (EELS). Boron doping was found to downshift the positions of the optical absorption bands associated with the van Hove singularities (E11s E22s and E11m) by ∼30 meV relative to their positions in acid-treated and annealed SWNTs. Raman spectroscopy, EELS, and optical data are consistent with the picture that a few atom % boron has been substituted for carbon in the sp2 framework of SWNTs. Finally, our results show that boron doping does not significantly affect the optical transmittance in the visible region. However, boron doping lowers the sheet resistance by ∼30% relative to pristine SWNT films from the same batch. Boron-doped SWNT may provide a better approach to touch-screen technology.
Electrically conducting and optically transparent networks of carbon nanotubes (CNTs) have stirred practical interest for applications in transparent electrodes, touch screens, and solar cells.1–5 Single-walled carbon nanotubes (SWNT)6 derived from large batch processes, such as the arc or plasma chemical vapor deposition, are composed of bundles of tubes with a somewhat random distribution of chiral integers (n,m) in which approximately one-third are metallic and approximately two-thirds are semiconducting nanotubes. Each filament in the percolating network can be comprised of bundles of 10s to 100s of SWNTs, as is the case in this study, or the filament might be an individual tube. Here we present results on how to improve the figure of merit, M ) %T/R0, of these percolating bundled SWNT films via boron doping, where %T is the optical transmission (e.g., at 550 nm for visible applications) and R0 is the sheet resistance. To the best of our knowledge, this work also represents the first attempt to confirm B-doping in SWNTs by a direct measurement of the resistance of films of such tubes. Chemical doping of various forms of sp2 carbon have been the subject of considerable interest over the past 60 years beginning with graphite intercalation compounds. In the early 1970s, the chemical doping of graphite by a reaction with AsF5 produced an intercalation compound with an electrical conductivity better than that of copper.7 In chemical doping, the dopant chemisorbs on the carbon surface, exchanges charge with the carbon, creating free carriers in the sp2 network. For charge neutrality, either a molecular anion (p* Corresponding author. † Department of Physics. ‡ Department of Materials Science. 10.1021/nl0729734 CCC: $40.75 Published on Web 08/02/2008
2008 American Chemical Society
doping) or cation (n-doping) is produced. The startling discovery in AsF5-graphite was followed by chemical doping studies in carbon fibers.8 Later similar studies in the more “modern” forms of sp2 carbon, i.e., fullerenes and single-walled carbon nanotubes, were carried out.9 The present work emphasizes the conductivity effects of substitutional doping in SWNTs, where the dopant must substitute for carbon in the sp2 lattice. Boron and nitrogen are the only elements that can be incorporated into an sp2 carbon network without significantly affecting the atomic arrangement in the hexagonal two-dimensional lattice. Early studies have shown that boron can be substituted at the few % level into the graphene sheets in graphite.10 Thus it is reasonable to expect that CNTs can be made p-type by substituting boron for carbon in the nanotube wall.11 In fact, B-doping has been calculated to downshift the Fermi level (EF) by ∼0.7 eV into the valance band of semiconducting (s) SWNT for ∼1-2 atom % boron content.12,13 Of course, the details of the effect of B substitution on electronic states depends on the chirality and diameter of the SWNTs. The calculations nevertheless suggest that boron doping of large batch SWNTs will transform the original semiconducting (s) fraction ∼(2/3) into metallic nanotubes, while the originally metallic (m) tubes (∼1/3) should remain highly conducting. Previous experimental studies regarding the possibility of B-doping in SWNTs have been made.11,14–22 Boron substitution has been reported to occur during11,14–16,22 or after17–21 the growth of the SWNTs. After growth of a perfect tube (no defects), the high stability of the CdC bond should make it difficult to substitute a boron atom. Therefore, postgrowth
B-doping methods probably involve, as a first step, the removal of a C-atom, e.g., as CO or CO2 to create or increase the number of wall defects. In a second step, a reaction of the dangling C-bond or defect with a suitable boron precursor occurs depositing B in the sp2 lattice of the tube wall. In this work, we obtained B-doped SWNTs by a postgrowth high-temperature chemical reaction of arc-derived SWNTs in direct contact with B2O3 in flowing NH3.17 Studies of the electrical conductivity and optical transmission of thin films of SWNT networks were made before and after B-doping and allow us to investigate the impact of B-doping. A model of percolating SWNTs is presented to understand the data. In this paper, we are not reporting the lowest sheet resistance in SWNT networks. Indeed, there are previous reports with much lower sheet resistance, i.e., 30 Ω/0 with more than 70% transmittance in the visible spectrum23(2.3), 120 Ω/0 with 80%24 (0.67), and 160 Ω/0 at 87% transparency25 (0.54), where the values in parentheses are the calculated values for the figure of merit, M ) %T/R0. In some cases, values of the two-probe resistance rather than sheet resistance are quoted in the literature. The sheet resistance can be higher or lower than the two-probe resistance, depending on the electrode geometry. A true sheet resistance measurement is to be preferred. In our case, we are using processed arc tubes still containing a high concentration of amorphous carbon (aC) and other impurities that affect the electrical properties of the overall sample. Our main goal here is to demonstrate the effect of boron doping on the optical transmission and sheet resistance of these thin films of SWNT networks relative to the processed material. The SWNT material investigated here was obtained from CarboLex, Inc., and is stated to consist of ∼50-70 vol % carbon as bundled SWNTs. The tubes were produced by an arc plasma discharge using a Ni/Y catalyst. The Raman spectrum (514.5 nm excitation) was found to be typical of high quality arcderived tubes, i.e., weak D-band scattering, a Raman radial band centered at ∼160 cm-1, and a stronger tangential band at 1592 cm-1.26 Typical high-resolution transmission electron microscope (HRTEM) images on similar CarboLex material have shown that the arc-tubes are present in bundles ∼3-5 µm long, with bundle diameters in the range 10-15 nm, i.e., typically containing ∼100-400 tubes.27 The SWNTs themselves have diameters in the range 1.2-1.6 nm.27 In this study, the as-prepared (AP) SWNT material from CarboLex was subjected to a mild three-step postsynthesis process to reduce both the aC and the metal catalyst content: (step 1) reflux in 3 M HNO3 for 48 h, (step 2) neutralization with a NaOH solution, followed by (step 3) filtration and washing with deionized water. After this treatment, we refer to the material as “processed”, and we identify this processed arc material as P-SWNT. The metal content in the acidtreated material was determined by ash analysis (combustion in dry air) to be ∼15 wt % catalyst residue (TA Instruments, Inc., model IR5000). This value is somewhat high but should not impact the sheet resistance which is dominated by percolating SWNT bundle networks. The P-SWNTs were 2614
then mixed with powdered B2O3 (grain size ∼10 µm) and placed in a quartz boat for the high temperature reaction. The weight fraction of B2O3:SWNT in the boat was ∼5. The mixed SWNT-B2O3 powder was heated at 900 °C for 4 h in a quartz tube reactor with flowing NH3 (110 sccm at 200 mbar) as the carrier gas.28 The product was then washed and filtered three times (polycarbonate membrane, 1 µm) using hot deionized water (100 °C) to remove residual B2O3. To prepare uniform SWNT films for optical and electrical measurements, P-SWNT and boron-reacted SWNT powders indentified here as (B-SWNT) were first sonicated in isopropanol (AE Spectral grade) using a probe sonicator (Misonix Inc., model 2000) for 1 h at 30 W. It should be noted that aggressive sonication must be done carefully, as it may cut the nanotubes into shorter segments.29 This cutting is not desirable for touch screen applications because longer tubes will percolate at lower density. Thin films of SWNTs were prepared by spraying the isopropanol-SWNT suspension onto clean quartz or ZnSe substrates via short bursts of air with a small air brush (Paasche Inc., model VL#1), such as used to paint fabric. Patterned films were made by spraying through a 3 × 3 mm2 square hole in a stainless steel mask. The number of bursts was used to gain some control over the film thickness. The substrates were maintained at 120 °C during the spraying process to rapidly evaporate the isopropanol. The SWNT films on quartz were then subjected to a final high-temperature vacuum-degassing (and anneal) at 1100 °C at 7 × 10-7 Torr for 12 h. This treatment is expected to remove solvent and functional groups attached to the tube walls.30 The films on ZnSe could not be heat treated at high temperature because of substrate decomposition. Therefore, the SWNT material was first subjected to high-temperature (1100 °C) vacuum anneal before being spray deposited. Then they were heated in air at 120 °C for 2 h to remove residual solvent and other physisorbed gases. The effects of the B2O3 reaction on the morphology of the SWNTs as well as the atom % boron-doping were studied by transmission electron microscopy (TEM) and core level electron energy loss spectroscopy (EELS). The measurements were made in a JEOL-2010F microscope with a Gatan Enfina TM 1000 EELS system. EELS information on the hybridization state of both boron and carbon were obtained by studying the K-edge absorption of each element.15,31,32 The energy dispersion was 0.2 eV/channel, and the absolute energy loss scale was calibrated using the graphitic π* peak at 285 eV. Low-power (P < 2 mW) micro-Raman experiments were performed on the SWNTs in a Renishaw INVIA microRaman spectrometer with cooled CCD using an Ar ion gas laser (514.5 nm) focused onto ∼1 µm spot of the sample. Optical transmission spectra were collected in a double-beam UV-vis-NIR spectrometer (Perkin-Elmer, model Lambda 950). Absorption/reflection structure and loss derived from the substrate were removed from the data by dividing the transmission of the film/substrate by that measured for the clean substrate. Nano Lett., Vol. 8, No. 9, 2008
Figure 1. (a) Schematic percolation of a 2D nanotube network made of SWNT bundles. Four percolating pathways are shown (red). (b) Schematic view of van der Pauw contacts (A, B, C, D) to SWNT film.
Direct current sheet resistance measurements were made by attaching four copper leads to the four corners (A, B, C, D) of 3 mm × 3 mm films on quartz using silver paste (EMS Inc.,12630) (van der Pauw method;33 cf. Figure 1b). The sheet resistance measurements were made at room temperature in air using four combinations of current and voltage contacts via an integrated current source-DVM system (Keithley; model 2400). The sheet resistance R0 was calculated according to the relation33
(
exp -π
)
RAB,CD + RAB,CD + RCD,AB + RCD,BA + 4R0 RAD,BC + RAD,CB + RBC,DA + RBC,AD ) 1 (1) exp -π 4R0
(
)
where, for example, RAB,CD ) VAB/ICD and VAB is the voltage measured between contacts A and B while the current is injected at C and withdrawn at D (Figure 1b). In Figure 2, we show typical low-magnification TEM micrographs of bundles of P-SWNT (Figure 2a) and BSWNT material (Figure 2b). The TEM images show a web of entangled SWNT bundles, and numerous carbon-encased metal nanoparticles can be seen attached to the bundle walls. This carbon encasement unfortunately protected the metal core of many of the catalyst nanoparticles from acid attack during the processing. The P-SWNT bundle walls were observed at higher resolution (HR) to be reasonably free from aC coating. That is, the HRTEM images of P-SWNT bundle surfaces are similar to that shown in Figure 2c for B-SWNT material. From our HRTEM images, we can also conclude that the B2O3-NH3 process does not appear to introduce significant damage to the nanotube walls. Raman scattering, however, is a more sensitive probe in this regard. Below we will discuss the application of Raman scattering to the characterization of B-SWNTs. The bundle size distribution was measured by TEM. This was done before and after the B-doping process. We found that a typical bundle diameter was ∼15 nm and that 80% of the bundles had diameters in the range 10 nm < d < 20 nm. Most importantly, the B-doping process was not observed to create any significant change in the bundle size distribution. So, we do not expect that additional parallel percolation paths were formed as a result of doping-induced splitting of the bundles into smaller diameter bundles. Had this occurred, even at fixed optical density, we would expect a reduction in sheet resistance on decreasing bundle diameter. Nano Lett., Vol. 8, No. 9, 2008
It should be noted that we expect the sheet resistance R0 should be increased by an aC coating of the bundles via an undesirable enhancement in the bundle-bundle contact resistance. The carbon-encased metal nanoparticles, on the other hand, probably make a relatively unimportant contribution to the sheet resistance but can produce an undesirable reduction in the optical transparency. In the ideal material, these metal particles need to be stripped out by a more effective purification. For example, a selective oxidation of arc SWNTs, prior to acid treatment, in flowing dry air (∼400 °C for 15 min) has been shown30 to weaken the carbon coating on the catalyst particles to the degree that HNO3 or HCl reflux can be used to leach out the metal leaving hollow carbon cores behind. This selective oxidation step was not carried out here. In our EELS K-edge measurements, we were careful to find regions of the material where the TEM beam probed only clean bundles of SWNT, and with this care, contributions from B-doped aC were kept to a minimum. For example, the EELS spectrum in Figure 2d comes from the entire image shown in Figure 2c. EELS spectra from a few bundles were thereby obtained by using a selective area aperture of diameters ∼140 nm; this condition was preferred instead of a focused nanoprobe because the larger probe area increases the signal-to-noise ratio for the K-edge studies and minimizes tube wall damage. The EELS spectrum in Figure 2 captures the K-edge of both boron and carbon. From the shape of the boron K-edge at 188 eV, we can see structure corresponding to the 1s f π* and 1s f σ* preionization edges characteristic of sp2 hybridization.15,31,32 We therefore can conclude that the majority of the boron in the bundle has been incorporated into the sp2 nanotube carbon network. It should be noted that we found several examples of aC with B content higher than that in the tube walls, i.e., B atom % as high as 10 atom % was observed in some aC regions, whereas typical values of 1-3 atom % boron were found for clean bundles of B-SWNTs. The variation in boron observed from bundle to bundle is probably associated with inhomogeneous chemical contact between the P-SWNT and the B2O3. It should also be mentioned that we did not observe the EELS peak at 194 eV corresponding to B2O3 impurities.28 This indicates that the washing process in hot deionized (DI) water efficiently removes most of the B2O3 residuals. Because of D-band scattering, Raman spectroscopy can be a much more sensitive probe of nanotube wall disorder than TEM.26 The Raman D-band is activated in sp2 carbons by various forms of disorder in the carbon network, and the scattering intensity depends on the disorder-induced relaxation of the first-order q ) 0 Raman selection rule for crystalline material. Recent theories for the D-band attribute the scattering to a double resonance scattering process.34 In Figure 3, we display Raman spectra in the range ∼100-1700 cm-1 for a series of SWNT films. This region includes the contributions from radial and tangential first-order allowed Raman scattering and D-band scattering. The Raman spectra are labeled (a)-(e): (a) as-prepared (AP) CarboLex (APSWNTs); (b) AP-SWNTs after HNO3 reflux and neutralization to pH ) 7 (i.e., spectrum for P-SWNTs); (c) P-SWNTs, 2615
Figure 2. (a) TEM image of as-delivered bundled SWNTs. (b) TEM image of processed (P) SWNTs. The dark dots in (a) and (b) correspond to residual NiY catalyst particles. (c) HRTEM image of several B-SWNT bundles. (d) EELS spectrum of the area shown in (c).
Figure 3. Room temperature Raman spectra showing radial R-band (∼160 cm-1), D-band (∼1350 cm-1), and G-band (∼1590 cm-1) of (a) as-prepared (b) after HNO3 reflux (c) after HNO3 reflux NaOH wash to pH ) 7 and 1100 °C vacuum anneal (d) after B2O3-NH3 treatment and (e) after B2O3-NH3 treatment and subsequent 1100 °C anneal.
but after an 1100 °C vacuum anneal (2 h, 10-5 Torr); (d) B-SWNTs (no vacuum anneal); and Figure 3(e) B-SWNTs after 1100 °C anneal in vacuum (12 h, 10-5 Torr). Starting with the bottom Raman spectrum (a) in Figure 3, the AP-SWNT material can be seen to exhibit a radial breathing mode (RBM) band at ∼160 cm-1. The frequency of this band is linearly related to the inverse of the individual nanotube diameter.35 The tangential band (or “G-band”) is located at ∼1592 cm-1 and is also in good agreement with the literature.26 Weak D-band scattering can be observed at ∼1350 cm-1 in the AP-SWNT spectrum, from either aC or 2616
imperfect tube walls. Our experience with arc tubes subjected to various purification treatments30 indicates that HNO3 reflux can induce a substantial increase in the wall disorder (defects plus functionalization, i.e., -COOH), as observed by an increase in D-band scattering. The effect of the increase in wall disorder due to HNO3 reflux can be seen in spectrum (b) in the figure, i.e., increased D-band intensity and, furthermore, a line broadening for the RBM bands and G-bands. We have identified these changes in the Raman spectrum with the removal of C-atoms followed by the addition of functional groups (e.g., -COOH, -OH).30 The wall-disorder-induced broadening of the Raman bands can be seen by eye in (b). However, it was also quantified using a multi-Lorentzian line shape analysis. More important to the present study is the observation that a short-term vacuum anneal of P-SWNT, i.e., ∼2 h at 1100 °C in 10-5 Torr, significantly reduced the D-band scattering intensity induced by processing (by a factor of ∼10), as shown in spectrum (c). After the vacuum anneal, the D-band of P-SWNTs is similar to that of AP-SWNTs material from the vendor (cf. (a)). Spectrum (d) in Figure 3 refers to B-SWNTs before a high-temperature vacuum anneal. These tubes can be seen to exhibit a strong D-band. Recalling that our thermochemical B-doping takes place at 900 °C, had there been no reaction of B2O3 with the tube wall, we would have expected a passive high-temperature environment to reduce the D-band intensity in the P-SWNT material. However, the D-band was found to increase in strength after the thermochemical reaction with B2O3. Therefore, this D-band increase is interpreted as evidence that a B-reaction with the tube walls has introduced disorder. To further test the connection of increased D-band scattering with possible B-doping, we then vacuum annealed (1100 °C for 12 h) a B-SWNT sample. The Raman spectrum Nano Lett., Vol. 8, No. 9, 2008
Figure 4. Optical density of thin SWNT films on ZnSe: (a) P-SWNTs; (b) after B2O3-NH3 processing and vacuum anneal at 1100 °C.
for this material is shown in (e). Unlike the response of P-SWNT material to such a vacuum anneal, only a small reduction in the D-band scattering is observed after the thermal anneal of B-SWNTs. This suggests that the D-band scattering that remains in the B-doped sample is associated with B-substitution and not remaining C-atom defects. Furthermore, we can also conclude that the B is stable in the tube walls at the annealing temperature (1100 °C), at least over annealing times of a few hours. Our proposal of the connection between the D-band intensity and B-substitution is also consistent with earlier work in B-doped graphite, where a systematic increase in D-band intensity with increasing B-doping was reported.36 In Figure 4, we show the optical density OD ) (-log10(T)) vs photon energy for P-SWNT and B-SWNT films on ZnSe substrates. ZnSe was chosen to allow the transmission spectrum to be obtained throughout the visible and IR regions. The OD values for P-SWNT and B-SWNT films are shown in parts a and b of Figure 4, respectively. The actual spectra are shown in the insets together with a linear optical background (solid line) that we identify with the low energy tail of carbon π-π* electronic interband absorption. After subtracting this background from the data, the resulting data (thick black lines) exhibited three broad bands that were fitted to Lorentzians via the method of least-squares. The thin red solid line is the result of the fit of the data (background removed), and the fit has been displaced downward slightly for clarity. The individual Lorentzian components are also shown in the figure. The three bands observed in the top spectrum (a) for P-SWNTs are labeled according to the literature as Ejjs(m), where jj ) 11, 22 refers to transitions between the closest and next closest pair of van Hove singularities and s(m) refers to semiconducting (metallic) tubes.37 Strictly speaking, the absorption involves the production of the exciton associated with the van Hove singularity.6 The position (photon energy) of these exciton bands for P-SWNTs is in good agreement with the literature for bundled arc tubes.38,39 A single broad Lorentzian was used to fit each Ejjs(m) band. In reality, each broad band actually contains many unresolved contributions from various (n,m) tubes with similar diameter. The bottom spectrum in Figure 4 is the OD for a B-SWNT film. Similar to the top spectrum for P-SWNTs, three broad optical absorption peaks are observed and at similar energies. However, we found Nano Lett., Vol. 8, No. 9, 2008
that the effect of B-doping is to slightly downshift each of these broad Lorentzians by ∼30 meV relative to their position in P-SWNT material while no significant change was observed in the widths of these bands. It is possible that the Ejj bands we observe in the B-SWNT material might stem from absorption in unreacted P-SWNTs or from both B-doped and unreacted P-SWNTs. It is not possible for us to separate the contributions from these two types of tubes. It is reasonable to expect that large bundles of SWNTs thermochemically exposed to B2O3/NH3 result in tubes that are more strongly doped toward the outside of the bundles. However, we had no probe at our disposal that allows us to say for sure that primary doping occurs to the outer tubes in the bundle. If this preferential doping toward the external surface of a bundle were to actually occur, then the local doping per tube in the outer tubes in the bundle would be significantly higher than the value 1-3 atom % obtained from EELS which represents an average over the entire bundle. A new optical absorption peak can be observed at low photon energy in the B-SWNT film (Figure 4b). The peak is located by a single Lorentzian band (least-squares fitting) to be at 0.40 ( 0.02 eV. Borowiak-Palen et al.,28 who first used the B2O3/NH3 reaction to B-dope SWNTs, also observed this 0.4 eV band in their OD spectrum and identified it with BC3 tubes. They made the identification of this band with BC3 using their electronic density of states calculations for BC3. Calculations have also been made regarding the electronic structure of boron-doped SWNTs.12,17 They all indicate that B-doping at the few atom % level will significantly downshift the Fermi level (EF) in semiconducting tubes. For example, one calculation shows that a few atom % boron substitution downshifts the EF by ∼0.7 eV into a hybridized acceptor band. Without calculated electric dipole matrix elements, it is difficult to identify the states involved in the 0.4 eV absorption. However, if one requires vertical transitions (kconserving), it seems most reasonable to us to identify the 0.4 eV absorption band with transitions in semiconducting or metallic tubes involving hybridized boron-carbon bands split by 0.4 eV, with EF located in the highest partially occupied hybridized band. Considering all the results (i.e., EELS, Raman scattering, and optical absorption), we conclude that a thermochemical B2O3/NH3 reaction with bundled SWNTs under our conditions does indeed produce Bsubstituted SWNTs. We next turn to the results that document the change induced by B-substitution on the film transparency and sheet resistance. Optical and electrical measurements were each made on the same film. The % transmission (%T) was recorded at 550 nm with a spot size which just underfilled the 3 mm × 3 mm area of the SWNT film. Films of various average thickness were prepared by spraying, and their %T and sheet resistance (R0) were both measured at room temperature under ambient conditions. The results are shown in Figure 5 for P-SWNTs and B-SWNTs, where we correlate the data by plotting R0 vs %T. One would expect that thicker films would give rise to lower R0 and lower %T. Thus data 2617
bundle. Assuming (1) the electrically interconnected bundles form conducting paths as shown schematically in the Figure 1a and (2) the paths are in parallel with each other, then the sheet resistance for P-SWNTs and B-SWNT square films can be written as Rp ) RB )
RC +
nearest the origin should pertain to the thickest films (although we did not measure the thickness). Data in the figure are plotted as diamonds (P-SWNTs) and solid circles (B-SWNTs). From the data in the figure, we can see the effect of B-doping is to reduce the sheet resistance at fixed transparency. This effect is particularly significant for the thinnest films that are the most transparent. The data in Figure 5 also show that the trend of the sheet resistance for the P-SWNT and B-SWNT samples is to approach each other in the limit of thick or poorly transmitting films. In this limit, the benefit of B-doping is minimized. The sheet resistance R0 and optical transmission of P-SWNT films has been modeled as a parallel network of percolating SWNT bundles.40 Our approach to the modeling is similar to that of Gruener and co-workers.1 In Figure 1a, we show a schematic of a two-dimensional percolating nanotube bundle network in a plane. Four conducting paths have percolated between the electrodes shown in red. The nanotubes within a bundle are considered to be a set of parallel resistors. For simplicity, we ignore the narrow distribution in bundle diameters. According to the individual nanotube resistance r, we expect the following hierarchy: rs . rds . rm, where the subscript s, ds, and m refer, respectively, to pristine semiconductor (s), degenerately doped semiconductor (ds), and metallic (m) tubes. The resistance for each bundle is taken as the parallel combination of the various tubes in the bundle. Taking into account the resistance hierarchy, the bundle resistance for a processed bundle (but with no B-doping) can be written as rp ≈
rm nm
where rm is the resistance of one metallic nanotube and nm is the number of metallic tubes in the bundle. Similarly, for B-doped bundles, and using rds , rs, we can write the bundle resistance as rp ≈
rm rm nm + nds rds
where nds is the number of degenerately B-doped tubes per 2618
P
N RC′ +
∑r
B
N′
RC′
Figure 5. Transmittance (at 550 nm) and sheet resistance of B-doped SWNT and undoped P-SWNT films. Insert: Transmittance in the visible range for undoped SWNT P-films.
∑r
(2)
(3)
where RC and are, respectively, the total contact resistance between P-SWNT and B-SWNT bundle in a typical percolating path. The summation in eqs 2 and 3 is over the series of connections of bundles in a percolating path. The variables N and N′ are the number of parallel equivalent conducting routes between current contacts of the film. In the limit of weak optical absorption, it can be shown that the sheet resistance and optical transmission of a thin film can be related as40 R)
2πβ √T and β ) σopt ⁄ σdc C 1 - √T
(4)
where σdc is the direct current conductivity, σopt is the real part of the optical conductivity at the wavelength at which T is measured, and c is the speed of light. Equation 4 is used to fit our experimental data shown in Figure 5 by adjusting the single parameter β in a least-squares fit. This fitting is done using only data from the high transparency region (T > 0.2) which is most consistent with the weak absorption approximation used to generate eq 4. The fitted curves (solid lines) in the figure correspond to RP )
7170√T 1 - √T
RB )
4868√T 1 - √T
and
i.e. 2πβP ) 7170 Ω C
and 2πβB ) 4853 Ω C
where the superscripts P and B refer to processed and B-doped material, respectively. Thus we can consider the ratio of the β’s, i.e. βP ⁄ βB ) (σPopt ⁄ σBdc)(σBdc ⁄ σPdc)
(5)
From our fitting results βP/βB ) 1.48. We can extract the ratio of the dc conductivities from eq 5 if we assume that the doping affects primarily the free carrier adsorption confined to the mid-IR and longer wavelengths. We have limited experimental evidence supporting this assumption from transmission spectra collected on the same film before and after doping. In this case, we observed a decrease in the transmission at 0.4 eV associated with the new interband peak shown in Figure 4. However, we observed no significant Nano Lett., Vol. 8, No. 9, 2008
change in the visible region of the spectrum. Therefore, we use the approximation σpopt/σBopt in eq 5 which results in βP ⁄ βB = σBdc ⁄ σPdc ) 1.48
or a B-doping induced enhancement of the dc conductivity of the film by a factor of ∼3/2. Referring to our percolation model, eqs 2 and 3, and assuming that the number of percolating parallel paths is not affected by the doping, then we set N ) N′, and we must have
∑r R ′+∑r RC + C
P B
≈
3 2
To proceed further, one has to know the relative contributions to the numerator or denominator made by the contact resistances RC and RC′. For example, it is thought that the contact resistance dominates the percolating path resistance in nanotube films. If this is the case in our films, then doping can be said to have lowered the contact resistance between bundles by a factor of 2/3. One can argue that the barrier to charge transfer from bundle to bundle has been lowered as the bundle conductivity increases. But other explanations are also possible. Conclusions. Raman scattering, optical absorption, and EELS indicate that exposure of SWNT bundles to B2O3/ NH3 at high temperature substitutes boron into the carbon sp2 framework at a few atom % boron. The role of NH3 in this reaction is obscure. We have observed significant changes in the electronic properties of spray-deposited SWNT thin films based on these B-doped SWNTs. Boron doping is observed to produce a significant reduction in the sheet resistance at room temperature. An analysis of our data on sheet resistance vs optical transmission at 550 nm indicates that the B-doping has indeed raised the dc conductivity of the film by a factor ∼1.5. At this time, we do not know if the improvement is dominated by a decrease in the bundle-bundle contact resistance or is due to a combination of a decrease in contact resistance with a significant decrease in bundle resistance. We are planning future studies on the doping-induced change of the resistance of individual bundles to improve our understanding of the properties of these percolating films. Acknowledgment. This work was funded, in part, by the Department of Energy (DOE) 428-54 68NA. References (1) Gruner, G. J. Mater. Chem. 2006, 16 (35), 3533–3539. (2) Saran, N.; Parikh, K.; Suh, D. S.; Munoz, E.; Kolla, H.; Manohar, S. K. J. Am. Chem. Soc. 2004, 126 (14), 4462–4463. (3) Kaempgen, M.; Duesberg, G. S.; Roth, S. Appl. Surf. Sci. 2005, 252 (2), 425–429. (4) Bekyarova, E.; Itkis, M. E.; Cabrera, N.; Zhao, B.; Yu, A. P.; Gao, J. B.; Haddon, R. C. J. Am. Chem. Soc. 2005, 127 (16), 5990–5995. (5) Rowell, M. W.; Topinka, M. A.; McGehee, M. D.; Prall, H. J.; Dennler, G.; Sariciftci, N. S.; Hu, L. B.; Gruner, G. Appl. Phys. Lett. 2006, 88 (23), 233506. (6) Saito, R.; Dresselhaus, G.; Dresselhaus, M. S., Physical properties of carbon nanotubes; Imperial College Press: London, 1998; p xii. (7) Falardeau, E. R.; Foley, G. M. T.; Zeller, C.; Vogel, F. L. J. Chem. Soc., Chem. Commun. 1977, (11), 389–390.
Nano Lett., Vol. 8, No. 9, 2008
(8) Dresselhaus, M. S., Graphite fibers and filaments; Springer-Verlag: Berlin; New York, 1988; p x. (9) Dresselhaus, M. S.; Dresselhaus, G.; Eklund, P. C., Science of fullerenes and carbon nanotubes; Academic Press: San Diego, 1996; p xviii. (10) Hach, C. T.; Jones, L. E.; Crossland, C.; Thrower, P. A. Carbon 1999, 37 (2), 221–230. (11) Carroll, D. L.; Redlich, P.; Blase, X.; Charlier, J. C.; Curran, S.; Ajayan, P. M.; Roth, S.; Ruhle, M. Phys. ReV. Lett. 1998, 81 (11), 2332– 2335. (12) Barone, V.; Peralta, J. E.; Uddin, J.; Scuseria, G. E. J. Chem. Phys. 2006, 124 (2), 024709. (13) Buonocore, F. Philos. Mag. 2007, 87 (7), 1097–1105. (14) Redlich, P.; Loeffler, J.; Ajayan, P. M.; Bill, J.; Aldinger, F.; Ruhle, M. Chem. Phys. Lett. 1996, 260 (3-4), 465–470. (15) Gai, P. L.; Stephan, O.; McGuire, K.; Rao, A. M.; Dresselhaus, M. S.; Dresselhaus, G.; Colliex, C. J. Mater. Chem. 2004, 14 (4), 669–675. (16) McGuire, K.; Gothard, N.; Gai, P. L.; Dresselhaus, M. S.; Sumanasekera, G.; Rao, A. M. Carbon 2005, 43 (2), 219–227. (17) Fuentes, G. G.; Borowiak-Palen, E.; Knupfer, M.; Pichler, T.; Fink, J.; Wirtz, L.; Rubio, A. Phys. ReV. B 2004, 69 (24), 245403. (18) Golberg, D.; Bando, Y.; Han, W.; Kurashima, K.; Sato, T. Chem. Phys. Lett. 1999, 308 (3-4), 337–342. (19) Han, W. Q.; Bando, Y.; Kurashima, K.; Sato, T. Appl. Phys. Lett. 1998, 73 (21), 3085–3087. (20) Han, W. Q.; Bando, Y.; Kurashima, K.; Sato, T. Chem. Phys. Lett. 1999, 299 (5), 368–373. (21) Golberg, D.; Bando, Y.; Bourgeois, L.; Kurashima, K.; Sato, T. Carbon 2000, 38 (14), 2017–2027. (22) Blackburn, J. L.; Yan, Y.; Engtrakul, C.; Parilla, P. A.; Jones, K.; Gennett, T.; Dillon, A. C.; Heben, M. J. Chem. Mater. 2006, 18 (10), 2558–2566. (23) Wu, Z. C.; Chen, Z. H.; Du, X.; Logan, J. M.; Sippel, J.; Nikolou, M.; Kamaras, K.; Reynolds, J. R.; Tanner, D. B.; Hebard, A. F.; Rinzler, A. G. Science 2004, 305 (5688), 1273–1276. (24) Li, J.; Hu, L.; Wang, L.; Zhou, Y.; Gruner, G.; Marks, T. J. Nano Lett. 2006, 6 (11), 2472–2477. (25) Zhang, D. H.; Ryu, K.; Liu, X. L.; Polikarpov, E.; Ly, J.; Tompson, M. E.; Zhou, C. W. Nano Lett. 2006, 6 (9), 1880–1886. (26) Dresselhaus, M. S.; Dresselhaus, G.; Saito, R.; Jorio, A. Phys. Rep. 2005, 409 (2), 47–99. (27) Kim, U. J.; Gutierrez, H. R.; Kim, J. P.; Eklund, P. C. J. Phys. Chem. B 2005, 109 (49), 23358–23365. (28) Borowiak-Palen, E.; Pichler, T.; Graff, A.; Kalenezuk, R. J.; Knupfer, M.; Fink, J. Carbon 2004, 42 (5-6), 1123–1126. (29) Liu, J.; Rinzler, A. G.; Dai, H. J.; Hafner, J. H.; Bradley, R. K.; Boul, P. J.; Lu, A.; Iverson, T.; Shelimov, K.; Huffman, C. B.; RodriguezMacias, F.; Shon, Y. S.; Lee, T. R.; Colbert, D. T.; Smalley, R. E. Science 1998, 280 (5367), 1253–1256. (30) Furtado, C. A.; Kim, U. J.; Gutierrez, H. R.; Pan, L.; Dickey, E. C.; Eklund, P. C. J. Am. Chem. Soc. 2004, 126 (19), 6095–6105. (31) Serin, V.; Brydson, R.; Scott, A.; Kihn, Y.; Abidate, O.; Maquin, B.; Derre, A. Carbon 2000, 38 (4), 547–554. (32) Terrones, M.; Benito, A. M.; MantecaDiego, C.; Hsu, W. K.; Osman, O. I.; Hare, J. P.; Reid, D. G.; Terrones, H.; Cheetham, A. K.; Prassides, K.; Kroto, H. W.; Walton, D. R. M. Chem. Phys. Lett. 1996, 257 (5-6), 576–582. (33) Van der Pauw, L. J. Philips Res. Rep. 1958, 13, 1–9. (34) Pimenta, M. A.; Dresselhaus, G.; Dresselhaus, M. S.; Cancado, L. G.; Jorio, A.; Saito, R. Phys. Chem. Chem. Phys. 2007, 9 (11), 1276– 1291. (35) Chen, J.; Hamon, M. A.; Hu, H.; Chen, Y. S.; Rao, A. M.; Eklund, P. C.; Haddon, R. C. Science 1998, 282 (5386), 95–98. (36) Hagio, T.; Nakamizo, M.; Kobayashi, K. Carbon 1989, 27 (2), 259– 263. (37) Kim, U. J.; Liu, X. M.; Furtado, C. A.; Chen, G.; Saito, R.; Jiang, J.; Dresselhaus, M. S.; Eklund, P. C. Phys. ReV. Lett. 2005, 95 (15), 157402. (38) Saito, R.; Fujita, M.; Dresselhaus, G.; Dresselhaus, M. S. Phys. ReV. B 1992, 46 (3), 1804–1811. (39) Kataura, H.; Kumazawa, Y.; Maniwa, Y.; Umezu, I.; Suzuki, S.; Ohtsuka, Y.; Achiba, Y. Synth. Met. 1999, 103 (1-3), 2555–2558. (40) Hu, L.; Hecht, D. S.; Gruner, G. Nano Lett. 2004, 4 (12), 2513–2517.
NL0729734
2619
