Length, Bundle, and Density Gradients in Spin Cast Single-Walled

Sep 13, 2010 - (1) Therefore, their use as the active component in a wide variety of .... Under these conditions, the Str 37 value reaches a maximum o...
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Length, Bundle, and Density Gradients in Spin Cast Single-Walled Carbon Nanotube Networks Qinghui Zhang, Pornnipa Vichchulada, and Marcus D. Lay* Department of Chemistry and NanoScale Science and Engineering Center (NanoSEC), UniVersity of Georgia, 1001 Cedar Street, Athens, Georgia 30602 ReceiVed: June 25, 2010; ReVised Manuscript ReceiVed: August 18, 2010

The ability to form single-walled carbon nanotube (SWNT) networks of known density and length distributions is of critical importance to the development of a wide variety of electronic materials that incorporate these molecular wires. Room temperature deposition methods are of great interest as they allow the use of heatsensitive substrates like polymers and glass. Additionally, they allow greater control over the characteristics of the SWNTs in the network by facilitating physical and/or chemical modification steps prior to deposition. This manuscript describes how two-dimensional networks of SWNTs were formed from aqueous suspensions via spin-casting, a deposition method commonly used in the microelectronics industry due to its ability to deposit uniform thin films of polymers from organic solvents. In the current work, the low viscosity of aqueous suspensions and high spin rates used resulted in ultrathin layers of aqueous suspensions from which low densities of unbundled SWNTs were deposited in iterative deposition cycles. This process was repeated to grow networks exhibiting tunable macroscopic conductivity that could be described by percolation theory. Introduction Single-walled carbon nanotubes (SWNTs) possess many unique and remarkable electronic properties due to quantum confinement effects induced by their size and curvature.1 Therefore, their use as the active component in a wide variety of electronic materials is of great interest.2-6 A major impediment to the widespread use of SWNTs in electronic applications is the dearth of manufacturable methods for the formation of low-density SWNT networks. Low-density networks are of significant technological interest because they can be tuned from semiconductive to metallic behavior as the density of SWNTs increases. Consequently, numerous methods of forming such thin films have been investigated. High-temperature methods usually involve growing them in place using chemical vapor deposition (CVD).7,8 Low-temperature, suspension-based deposition methods are of great interest because they allow the use of a wider array of substrates, as well as the inclusion of various SWNT purification/modification steps. Therefore, numerous room-temperature deposition methods for the formation of SWNT thin films have been developed, including spray-coating,9,10 dip-coating,11 spin-coating,12 suspension evaporation,13,14 and layer-by-layer assembly.15 While there are utilities for each of these methods, they all allow SWNT bundle formation during the deposition process. Bundle formation results exclusively in SWNT networks that exhibit metallic behavior. Laminar-flow deposition (LFD) is a method that allows the formation of low-density networks as isolated SWNTs are deposited at a rapidly receding solvent front. Since the density of SWNTs in these networks can be strictly controlled,16,17 these deposits exhibit semiconductive behavior at low densities and metallic behavior at high densities,18,19 due to the 1:2 ratio of metallic to semiconductive SWNTs.20 This paper describes the formation of macroscopic two-dimensional SWNT networks via iterative deposition cycles from a suspension of unbundled SWNTs. Spin-casting is a significant step forward in under* Corresponding author, [email protected].

standing the LFD process due to the increased level of scalability inherently provided by this deposition method. Additionally, a much greater level of understanding over the deposition process is facilitated by the ability to determine the thickness of the residual solvent layer from which SWNTs are deposited, as the factors involved in dictating the thickness of this layer are well understood.21 The low viscosity of water and decreased surface tension (due to the surfactant used to facilitate SWNT dispersion) enable even spreading of the suspension during spinning. Then the high centripetal acceleration results in thinning the suspension to a submicrometer thickness, where rapid evaporation occurs. The result is the deposition of a low density of unbundled SWNTs in each deposition cycle. A controllable change in density, average SWNT length, and macroscopic conductivity was observed for these networks at a variety of distances from the center of rotation, r. Experimental Details Formation of SWNT Suspensions. As-produced (AP) grade arc discharge soot (Carbon Solutions, Inc.) was dispersed in an aqueous solution of 30 mM sodium dodecyl sulfate (SDS, J.T. Baker) by 30 min of probe ultrasonication at 12 W (Fisher, model 500).22 By use of a previously described method, these suspensions were purified via multiple 45 min centrifugation cycles at 18000g (Beckman, Microfuge), in order to remove impurities such as metal catalyst nanoparticles, SWNT bundles, and amorphous carbon.23,24 Following each centrifugation cycle, 80% of the supernatant was carefully decanted and used for the next purification cycle. This purification method formed suspensions of unbundled, high aspect ratio SWNTs without the use of oxidizing acids that damage and shorten the nanotubes.19 Substrate Preparation and SWNT Network Formation. In order to form reproducible contacts to the SWNT networks, Ti (Kurt J. Lesker Company, 99.995% pure, 1/8 in. diameter pellets) was vapor deposited through a homemade stainless steel stencil using a dual filament thermal evaporator (Thermionics

10.1021/jp105884e  2010 American Chemical Society Published on Web 09/13/2010

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Figure 1. AFM images were analyzed for each deposit at various distances from the center of rotation, r: (a) 8 × 8 µm AFM image of a low density network of spin-cast SWNTs; (b) r had no observable effect on the orientation of SWNTs, as shown by autocorrelation analysis.

Laboratory) operating under high vacuum (P < 1 × 10-6 Torr). These electrodes formed a square pattern of 2 mm diameter dots having a 1.0 cm spacing. All distances, r, were referenced from the center of each 2 in. wafer to the center of the electrode pairs. Prior to SWNT deposition, a silane adhesion layer was formed on the wafer by a 45 min immersion in a solution of 100 µL of (3-aminopropyl)triethoxysilane (99%, Aldrich) in 20 mL of ethanol (99.5%, absolute 200 proof, ACROS). Then in order to ensure that only one monolayer of the silane remained, the wafers were cleaned with ethanol, water, and a CO2 jet, in that order. For network formation, 500 µL of SWNT suspension was drop cast onto the center of a 2 in. Si/SiOx wafer, while it rotated at 6000 rpm. After an 80 s drying period, the substrate was rinsed with two 1 mL aliquots of nanopure water to remove residual surfactant. This process was repeated for all samples as described below. Characterization Methods. The morphology of the SWNT networks was characterized by an atomic force microscope (AFM, Molecular Imaging, PicoPlus) operating in intermittent contact mode under ambient conditions. Image resolution, scan size, and scan rate were 512 × 512, 8 µm2, and 0.4 lines/s, respectively. To determine the effect of distance from the center of rotation on the degree of SWNT alignment, height, length, and coverage, scanning probe image analysis software (Image Metrology, SPIP v5.1) was used to evaluate all images. A probe station (Signatone, S-1160A) and semiconductor characterization system (Keithley, 4200SCS) were used for macroscopic electronic characterization. Confocal Raman microscopy (ThermoFisher, DXR Smart Raman Microscope) was performed on all samples using a 532 nm laser excitation source, 10× objective, and 10 mW power at the sample. Results and Discussion Deposition of Randomly Oriented SWNT Networks. At least five AFM images were obtained for distance (as described below) from the center of rotation, r. These images were analyzed in order to determine the effect of r on the degree of alignment, extent of bundle formation, average length, and the density of SWNTs deposited (Figure 1). The Str 37 function allows quantization of the degree of alignment of SWNTs. It works by assigning autocorrelation centers to areas on the surface with a height greater than 0, the height of the substrate.

Next, an autocorrelation function is applied to the height data in all horizontal directions away from each of these centers. Then, Str 37 is defined as the distance of the boundary where the ratio of height to distance from each autocorrelation center decays to 37% of the value at the autocorrelation center. For a low density of an anisotropic adsorbate, such as SWNTs, the rate of decay is exponentially greater orthogonal to versus along the nanotubes axis. Then the correlation length of each of these boundaries is plotted on a hemispherical histogram of correlation length versus angle. The graph for a surface composed of randomly oriented nanowires is composed of filled bins in all directions (Figure 1b), as the fibers on the surface only correlate well with themselves at the middle of each center and there is no dominant angle of anisotropy in the height measurements. Under these conditions, the Str 37 value reaches a maximum of unity. If there is a peak present, it points in the direction of dominant nanowire alignment.16,18 However, data obtained at numerous distances from the center of rotation for many samples indicated that distance had no noticeable effect on the degree of SWNT alignment, as randomly oriented SWNTs were routinely observed over the entire surface. Effectiveness for the Deposition of Unbundled SWNTs. The formation of suspensions of unbundled SWNTs was found to be a strict requirement for the subsequent deposition of lowdensity networks of unbundled SWNTs.18,23 When such suspensions are used, the deposition of low-density networks of unbundled SWNTs was found to be possible if a rapid drying technique is used.25,26 This occurs because rapid drying allows the iterative deposition of low densities of SWNTs, while avoiding bundle formation. Spin-casting is a significant step toward obtaining reproducibility and scalability in the formation of such networks, as it allows the determination of the ideal physical conditions under which unbundled SWNTs can be deposited with known density. Height histograms of AFM images formed from spin-cast SWNT networks indicated that each deposition cycle left behind a low density of unbundled SWNTs (Figure 2a). The average height observed was ∼1.4 nm, consistent with the height of an individual arc discharge grown SWNT. However, an increase in the mean and variance of height histograms obtained near the circumference of the wafers indicated an increase in the density of bundles (Figure 2b). This increase in the density of bundles can be ascribed to the presence of nonideal deposition

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Figure 2. Height analysis of AFM images: (a) height histograms obtained from AFM images were consistent with the presence of unbundled SWNTs (∼1.4 nm high) for most of the surface; (b) height histograms at various radii indicated that there was a significant increase in the occurrence of SWNT bundles near the outer perimeter of the wafer, at r ) 2 cm.

conditions as the suspension pooled near the edge of the wafer prior to expulsion. This allowed small bundles of SWNTs to form preferentially at the edge of the wafer. Variations in SWNT Length with Distance from the Center of Rotation. Deposits formed via three deposition cycles were used to determine whether centripetal forces caused any length distributions in the films. While macroscopic conductivity did not exist under these conditions, the density of SWNTs was low enough to facilitate the determination of the lengths and densities of SWNTs deposited via AFM image analysis. Because the magnitude of centripetal force on an object is directly proportional to its mass, concentration gradients driven by centripetal forces are more likely to develop for longer nanotubes. These high aspect ratio SWNTs were driven away from the center of rotation at the very short times when shear forces dominated solvent movement. However, low aspect ratio SWNTs were more likely to be evenly distributed across the surface. Then at longer times, when evaporation dominated the solvent movement in the thin layer left behind, any remaining SWNTs were deposited. Consequently, as high aspect ratio SWNTs experienced greater inertial forces, they were more likely to be swept along with the large solution front created during the initial stages when shear forces dominate. Although the minimum lengths of individual SWNTs was 63-74 nm for all distances, the upper limit of lengths varied significantly, reaching a maximum of 5.487 µm at r ) 2.0 cm (Table 1). The increase in SWNT length with distance from the center of rotation is responsible for the increase in the standard deviation of the measurements (Figure 3). The maximum SWNT lengths were observed for r ≈ 2.0 cm. These increased lengths coincide with the increased observance of bundles, as determined from height information. Therefore, these longer nanowires were most likely not individual SWNTs, indicating nonideal deposition conditions at the circumference of the wafer. Overall, the average length of SWNTs remained around 1.00 µm for distances between 0.5 and 1.5 cm. At distances between 0 and 0.5 cm, the distribution of lengths was most closely

TABLE 1: After Three Deposition Cycles, the Shorter SWNTs Were Evenly Dispersed across the Substrate, While the Occurrence of Longer SWNTs Was Greater at Larger Distancesa radius (cm) no. of SWNTs length range (nm) average length (nm) 0.0 0.5 1.0 1.5 2.0

107 ( 11 99 ( 10 113 ( 10 92 ( 16 106 ( 7

66-3175 66-4683 63-3750 74-4092 66-5487

726 ( 130 1248 ( 180 1104 ( 300 979 ( 520 1781 ( 710

a The number of SWNTs remained relatively consistent over the entire wafer, so the overall increase in standard deviation with distance is due to the increase in the upper range of SWNT lengths.

Figure 3. The precision of the length measurements was observed to decrease with greater distance from the center of the wafer due to the greater occurrence of longer SWNTs.

clustered about the mean. This results from the fact that low aspect ratio SWNTs were observed in similar densities across the entire surface, although a density gradient was observed for high aspect ratio SWNTs, with length increasing commensurate with distance, r. This resulted in a decrease in the precision of the length measurements (as observed by an increase in standard deviation). At distances above 1.5 cm, a sharp increase in average length and standard deviation was observed. This was

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R ) ω2r ) (628.32 rad/s)2(0.015 m) ) 5922 m/s2

Figure 4. Raman spectroscopy indicated that the density of defects on the SWNTs was constant across the wafer. Therefore, the length and bundle gradients observed were most likely caused by centripetal forces.

Figure 5. The average coverage of items greater than 1 nm in height was 2.2%, excluding observations near the circumference of the wafer, where edge effects resulted in increased deposition. With the exception of the outermost measurements, the coverage calculations were found to agree at 95% confidence intervals.

due in large part to the increase in density of high aspect ratio SWNTs at the outer edges of the wafer. Above 2 cm, edge effects became more pronounced. This is because the suspension pooled at the wafer’s edge until each drop was massive enough to be expelled from the surface. This allowed slightly more time for SWNT deposition to occur, resulting in less precise results. Confocal Raman spectroscopy was used to determine if the length separation observed was driven by any variations in defect density on the SWNTs. Raman scattering has been widely used as a probe for disorder in the carbon skeleton of sp2 and sp3 hybridized C materials.27 The graphite band (G-band), centered at 1591 cm-1, is indicative of a pristine, sp2 hybridized system,28 while the presence of a disorder-induced band (D-band) at 1340 cm-1 is a strong indicator of defects in the graphene lattice. Apparently, the density of defects on nanotubes of various lengths played an insignificant role in the observed length gradients, as Raman spectra showed that the ratio of the G-band to the D-band changed little with r (Figure 4). Variations in Coverage with Distance. To understand the effect of the spin-casting process on the coverage of material deposited, the total coverage of all material greater than 1 nm in height was evaluated at various distances. The 95% confidence intervals for five replicate measurements at each distance indicated a largely uniform distribution of material (Figure 5). The minimum standard deviation for the measurements was observed at r ) 1.5 cm. This indicates that when the acceleration due to the centripetal force

optimum deposition conditions were achieved. Therefore, although conditions under which unbundled SWNTs may be deposited exist over the entire surface, greater precision in the deposition process is obtained around r ) 1.5 cm. Closer to the circumference of the wafer, an increase in coverage sufficient to make this datum an outlier at the 95% confidence interval was observed. This increase was due to greater densities of SWNTs, as well as an increase in the density of impurities observed. These observations can again be ascribed to edge effects resulting in the excess deposition of SWNTs and any impurities remaining in the deposition suspensions. A Closer Look at the Deposition Process. The variations in SWNT length can be explained by consideration of the competition between inertial and viscous forces at sufficiently high angular velocities.21,29 As excess suspension was dispensed in order to achieve complete wetting of the surface, shear forces within the suspension dominate at short times. Initially, solution flow away from the center of rotation is the dominant force that affects the solution volume (Figure 6a). This causes a rapid depletion of solution volume over the entire wafer, starting with points near the center of rotation. During this initial period, the evaporation rate is slow compared to the suspension flow rate. However, at longer times, as the suspension volume becomes low, the viscosity effects at the domain boundary between the suspension and the substrate dominate, and the shear forces caused by centripetal acceleration become comparatively unimportant (Figure 6b). During this period, the suspension forms a thin, evenly distributed coating that is co-rotating with the surface. Due to the low viscosity of water, the volume of solution remaining at this stage is small with respect to the SWNTs, confining the unbundled nanotubes to a thin layer just prior to their deposition (Figure 6c). Then, as solution evaporation is the dominant factor driving the mass transport of solvent molecules at this stage, any unbundled SWNTs remaining in this small volume of suspension are deposited during the drying process. Two Methods to Estimate the Solvent Layer Thickness. During the spin-coating process, the thickness of the solvent layer during the evaporation phase plays a crucial role in determining the amount of colloidal fibers that will be deposited. In order to obtain an estimate of this thickness, the following equation was used21

h)

h0



(1)

4Fω2h02t 1+ 3η

where h0 ) the initial film thickness (calculated assuming a uniform coverage of 500 µL on a 2 in. wafer) ) 2.47 × 10-4 m, F ) the solvent’s density ) 1007 kg/m3, ω ) spin rate ) 628.32 rad/s, η ) solvent viscosity ) 3.85 × 10-3 Pa s30 and t ) time for solvent evaporation of thin layer, 15 s. The thickness calculated in this manner is 700 nm. This assumes an initially homogenously distributed suspension, resulting in h0 ) 2.47 × 10-4 m. If this is not the case, the solvent layer thickness during the evaporation dominate phase will be smaller. An alternative method to estimate the thickness of the suspension layer from which the SWNTs are deposited is possible, as the density of SWNTs deposited in each deposition cycle can be determined from analysis of AFM images and the

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Figure 6. Representation of the deposition process: (a) shear forces dominate as excess suspension is dispersed to achieve complete wetting; (b) evaporation becomes the driving force behind mass transfer as viscosity effects dominate when the suspension volume becomes low; (c) then, as evaporation of the thin layer of suspension occurs, low densities of SWNTs are deposited on the substrate.

density of the suspension from which the SWNTs are deposited is known. Extensive analysis of AFM images in these studies demonstrated that SWNTs were deposited at a rate of 0.63 SWNT/µm2 per deposition cycle. If one assumes an average SWNT diameter of 1.5 nm, length of 997 nm, and C-C bond length of 0.142 nm,31 then the mass of the average SWNT is 6.2 × 10-18 g and 6.0 × 10-9 g/m2 of SWNTs are deposited in each deposition cycle. The concentration of the SWNT suspension was 25.0 g/m3. Therefore, a volume of 2.4 × 10-10 m3 of suspension was needed to deposit SWNTs at the observed rate, assuming all SWNTs in the thin layer of suspension were deposited in each deposition cycle. For a 2 in. wafer, this corresponds to a suspension thickness of 120 nm. As this model assumes that all SWNTs in the thin solvent layer are deposited each time, the true value of the thickness would be greater if this assumption is untrue. Therefore, the actual thickness of the solvent layer during the deposition process falls between 120 and 700 nm. Since the suspensions were composed of unbundled, high aspect ratio SWNTs, the small magnitude of the thickness of the solvent layer from which they were deposited played a critical role in the deposition of unbundled SWNTs. The low viscosity of the solvent and high spin rates used created conditions under which such a thin layer, with nanometer-scale thickness, could form. Variations in Macroscopic Conductance with Distance. As the number of deposition cycles increased from 15 to 25, there was a commensurate increase in conductance across each wafer (Figure 7). The magnitude of conductance consistently decreased with r because although the density of SWNTs in the suspension remained constant, the area over which they were deposited increased with r. Therefore, as the increase in area of the wafer can be best fit to the power law equation πr2, this rate of increase in surface area is sufficient to overcome the advantage of having slightly longer SWNTs at larger r. As the number of deposition cycles increased, the response of the macroscopic conductance with respect to distance began to fit a linear regression, indicating increased precision in macroscopic electrical response with greater numbers of SWNTs deposited. Percolation Theory and the Critical Exponent. Percolation theory allows one to predict the macroscopic behavior of a system composed of micro- or nanoscopic components.17,32-35 In the case of SWNTs, macroscopic conductivity occurs when the nanotube density (N) exceeds the critical density (Nc). This was observed to occur after 14 depositions cycles. Plots of conductance versus the density of nanotubes above the critical

Figure 7. The variations in macroscopic conductance vs distance from the center of rotation after 15, 20, and 25 deposition cycles fit linear regression curves.

Figure 8. Power law curves calculated for the change in conductance between pairs of Ti electrodes at various distances from the center of rotation on a Si/SiOx wafer indicated an exponential dependence of conductance on the density of SWNTs.

density (N-Nc) were best described by power law equations, each having a critical exponent, R (Figure 8). This indicates that macroscopic conductance is largely density dependent, increasing exponentially with increasing densities of SWNTs, above Nc. While previously reported theoretical studies have indicated that R ≈ 1.33 for conducting sticks,36 experimental studies with SWNTs have consistently yielded a greater value.37-39 The

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Figure 9. A plot of the average critical exponent (R) vs r shows a minimum value is obtained at r ) 1.32 cm.

theoretical value of R describes a system at or near Nc, composed of conducting sticks of the same length and conductivity. Therefore, the discrepancy between theoretical and experimental data may be attributed to the existence of Schottky barriers between metallic and semiconductive nanotubes, as well as variations in the length, band gap, and the electrical conductivity of SWNTs. Additionally, deviations from ideal behavior are expected as the density of SWNTs increases above Nc. A plot of the average critical exponent observed versus r was used to ascertain the effect of radius on the degree to which these networks conform to the ideal conditions used to derive percolation theory. This plot was best fit to an upward facing parabola (Figure 9), with the simplified quadratic formula

R ) (r - 1.32)2 + 2.22

(2)

This equation specifies that R reaches a minimum, 2.22, at a distance of r ) 1.32 cm. This is in agreement with the earlier observation that the variance of height histograms and coverage measurements reached a minimum value between 1.0 and 1.5 cm. The deviation from ideal behavior observed at inner regions can be attributed to turbulent flow likely occurring near the center of the wafer when the drop cast suspension contacted the rotating substrate. At outer regions of the samples, edge effects caused by the suspension pooling into droplets that were massive enough to be expelled from the edge of the wafer resulted in nonideal deposition conditions. Conclusion Two-dimensional networks of unbundled SWNTs were spincast from aqueous suspensions. The thin layer of suspension formed during the spinning process led to conditions under which low densities of unbundled high aspect ratio SWNTs could be deposited. Repetition of the deposition process showed that the macroscopic conductivity of these networks could be tuned in a predictable manner. Bundle, length, and coverage gradients, driven by centripetal forces, were observed via AFM. Disruptions in the precision of the deposition process near the circumference of the wafer could be attributed to edge effects that disrupted the rapid laminar drying process. Percolation theory was applied to explain the macroscopic properties of these networks over multiple deposition cycles. The critical exponent varied across the wafer, achieving an

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