Controlled Two-Dimensional Pattern of Spontaneously Aligned

NANO LETTERS

Controlled Two-Dimensional Pattern of Spontaneously Aligned Carbon Nanotubes

2006 Vol. 6, No. 1 55-60

Robert S. Mclean,† Xueying Huang,†,‡ Constantine Khripin,§ Anand Jagota,§ and Ming Zheng*,† DuPont Central Research and DeVelopment, Wilmington, Delaware 19880, and Department of Chemical Engineering, Lehigh UniVersity, Bethlehem, PennsylVania 18015 Received October 1, 2005; Revised Manuscript Received November 4, 2005

ABSTRACT We report a simple solution process to form controlled patterns of aligned single-walled carbon nanotubes on solid substrates. The essential step of the process is to deposit a dilute solution of DNA-wrapped carbon nanotubes (DNA-CNTs) on a SiO2 surface covered with a thin hydrophobic layer. This leads to deposition of fully aligned CNTs. The alignment pattern can be controlled by metal electrodes in the deposition region and can be quantitatively modeled by the behavior of a quasi-two-dimensional DNA-CNT nematic phase near the solution/SiO2 interface. These results point to the possibility of rational design and economical fabrication of CNT alignment patterns on solid substrates.

Controlled assembly of nanoscale building blocks on solid substrates remains a great challenge in nanotechnology. This is especially true for carbon nanotubes, for which promising applications in fields such as nanoelectronics require wellaligned tubes on solid substrates.1 Many approaches for nanotube and nanowire alignment have been reported.2-7 Most carbon nanotube (CNT) alignment methods, such as those employed during in situ growth,5-7 rely on conventional photolithography. They are only suitable for making low density tube arrays and are not selective in tube types due to the lack of synthetic control. Motivated by the desire to develop a solution-based assembly process for CNT-based electronics, we have been investigating interfacial behavior of soluble CNTs on solid substrates. Here, we report our observation of spontaneous alignment of CNTs on solid substrates and controlled pattern formation using metal electrodes. To qualitatively explain the physical mechanism of the spontaneous alignment, and to quantitatively describe the geometric patterns of the controlled alignment, we propose that a quasi-two-dimensional (2D) CNT nematic phase forms near the solution/substrate interface. The solution alignment method we report here can be combined with the solution separation technique we have developed earlier8,9 to achieve controlled placement of CNTs with defined structure, bringing us one step closer to a true bottom-up and cost-effective strategy for CNT electronics. * Corresponding author. E-mail: [email protected]. † DuPont Central Research and Development. ‡ Current address: Sepax Technologies, Newark, DE 19711. § Lehigh University. 10.1021/nl051952b CCC: $33.50 Published on Web 11/23/2005

© 2006 American Chemical Society

Single-walled carbon nanotubes wrapped by singlestranded DNA (DNA-CNT) are well dispersed in aqueous solution,8,9 enabling many physical and chemical studies as well as biological applications of singly dispersed CNTs.10-13 As a starting point toward a solution-based assembly method, we have investigated in this work adsorption of DNA-CNTs from solution to inorganic substrates such as SiO2. Aqueous solution of d(GT)30-wrapped CoMoCAT CNT is prepared and purified as previously described,8,9,12,14 with an average tube length of ∼500 nm. In a typical experiment, a 10 µL drop of the DNA-CNT solution at a concentration of ∼1 µg/mL is deposited on the top SiO2 layer (100-150 nm thick) of a Si chip, or on a glass (Corning 7059) substrate. After 15 min of incubation at ambient conditions, the drop is rinsed away with deionized water and the substrate is dried with nitrogen gas. Atomic force microscopy (AFM) is then used to visualize DNA-CNTs adsorbed onto the substrate (Figure 1). A bare SiO2 surface is hydrophilic with ionizable Si-OH moieties (pKa ) 5).15 We found that at pH > 5, DNA-CNT does not adsorb on the substrate following the abovementioned deposition protocol, presumably due to the strong electrostatic and hydration repulsions between negatively charged DNA-CNTs and the similarly charged surface. At pH < 5, DNA-CNTs do adsorb, but with random orientation. To explore the effect of SiO2 surface charge density and its hydrophilicity on DNA-CNT adsorption, we coated an SiO2 surface with a hydrophobic layer using a commercially available chlorinated organopolysiloxane reagent (Sigmacote

Figure 1. Schematic showing steps involved in DNA-CNT solution deposition and adsorption experiments.

from Sigma, St. Louis, MO). When the same deposition experiment is done on the modified SiO2 surface, adsorption occurs over a broader pH range (4-9), and surprisingly, the adsorbed nanotubes are aligned parallel to each other. The alignment direction persists over the entire area covered by the drop, with a variation less than (20° (Supporting Information, Figure S1). The direction of the alignment is influenced neither by the rinsing flow direction nor by the N2 blowing direction, indicating that the alignment mechanism is different from that of the molecular combing phenomenon previously observed for double-stranded DNA16 and CNTs.17 It is important to note that noncrystalline glass also exhibits a DNA-CNT alignment phenomenon after treatment with Sigmacote. This observation excludes substrate crystal structure as the main cause of CNT alignment. To further elucidate the role of the hydrophobic layer on alignment, we prepared SiO2 surfaces covered with different alkyl chain monolayers of defined thickness. These are done by reacting the SiO2 surface with monochloroalkylsilanes, SiCl(CH3)2CnH2n+1, where n ) 2, 8, and 18 (Supporting Information, Figure S2). Such modified surfaces have advancing and receding water contact angles of ∼100° and ∼90°, respectively. The relatively large water contact angles with small hysteresis indicate that all three surfaces are very homogeneous and have similar hydrophobicity. However, they show marked differences in nanotube adsorption. For both C2 (-Si(CH3)2C2H5) and C8 (-Si(CH3)2C8H17) coated surfaces, we observed very uniform alignment of adsorbed DNA-CNTs. In contrast, when the C18 (-Si(CH3)2C18H37) coated surface is used, DNA-CNT adsorption occurs without apparent alignment. These results indicate that the thickness of the hydrophobic layer greatly influences the formation of an ordered DNA-CNT phase. Other factors influencing adsorption rate have also been studied using the C2 coated substrates. When DNA-CNT solution in a pH 7 buffer (10 mM Tris/0.25 mM EDTA) is used, the amount of adsorbed nanotubes increases monotonically with incubation time from 2 to 64 min. (Figure 2a-c). By varying the DNA-CNT concentration, we also found that the adsorption rate is proportional to the DNA-CNT concentration (Figure 2d-f). Adsorption of charged species on SiO2 is generally affected by solution pH due to changes in the protonation state of the remaining surface Si-OH groups after silanization and the resulting changes in electrostatic interactions. In light of this, we systematically varied solution pH to study its effect on DNA-CNT adsorption rate. As shown in Figure 2g-i, while nanotube alignment is maintained over the pH range of 4-8, the adsorption rate is substantially higher at lower pH. We also found that higher 56

Figure 2. Adsorption kinetics of DNA-CNT onto C2-modified SiO2 surfaces. Each panel is a 3 µm × 3 µm AFM image. (a-c) Nanotubes are in a pH 7 buffer (10 mM Tris/0.25 mM EDTA) at a concentration of 1 µg/mL. The amounts of deposition time are 2 (a), 8 (b), and 64 min (c), respectively. (d-f) Nanotubes are in a pH 7 buffer (10 mM Tris/0.25 mM EDTA) at a concentrations of 1 (d), 0.25 (e), and 0.06 µg/mL (f), respectively. Deposition time is held constant at 15 min. (g-i) Nanotubes at a concentration of 1 µg/mL are in 10 mM sodium phosphate/0.25 mM EDTA, at pH 4 (g), 6 (h), and 8 (i), respectively. Deposition time is held constant at 15 min.

ionic strength increases adsorption rate (data not shown). These results all suggest strongly that electrostatic interactions play an important role in the adsorption process. Controlling the direction of alignment is critical for applications. We noticed that if separate areas of one silicon chip are chosen for different DNA-CNT deposition experiments, the alignment orientation is always the same. (See, for example, Figure 2a-c.) This suggests that the Si substrate provides an easy axis for tube alignment, similar to the substrate boundary effect well-documented in 3D liquid crystals.18 However, the direction of the easy axis varies to some degree with surface treatment procedure in a way not well understood. A definitive way to control alignment directions is highly desirable. We hypothesized that nanotube alignment is the result of a quasi-2D nematic phase formed near the solution/substrate interface. If this is the case, then boundary lines of the 2D phase, the counterpart of boundary surfaces in 3D nematics, may be used to control tube alignment. To create such a boundary line, we conducted deposition experiments with patterned metal electrodes on the surface, knowing that the strong internal electrostatic field at the solution-metal interface may orient polarizable CNTs. Figure 3 shows two deposition experiments on a same Si chip with Sigmacote-treated SiO2 surface. In the first experiment (Figure 3a), a drop of DNA-CNT solution is deposited in an area covered on the left part by one Au electrode (0.8 mm × 0.8 mm × 50 nm Au plus 5 nm of Ti at the bottom for adhesion). Nanotubes near the metal Nano Lett., Vol. 6, No. 1, 2006

Figure 3. (a) Experimental setup and observed alignment pattern for DNA-CNT deposition with one Au electrode (thickness ) 50 nm) occupying the left part of the drop area. (b) Experimental (open circles) and simulated (solid line) tube tilting angles as a function of distance from the Au boundary. (c) Representative AFM images of the one-electrode experiment. (d) Experimental setup and observed alignment pattern for DNA-CNT deposition in the 500 nm gap between a pair of metal electrodes. (e) Experimental (open squares) and simulated (solid line) tube tilting angle as a function of distance from the left electrode. (f) Representative AFM images of the twoelectrode experiment. Statistical error bars in b and e are calculated based on the tilting angles of 50 randomly picked tubes at each position.

boundary are found to be tilted ∼22° from the horizontal (x) orientation. Away from the boundary, the tilting angle θ gradually increases to reach a constant value of ∼48° over ∼400 µm length and remains so well beyond (Figure 3b,c). Since the 48° tilting angle corresponds to the alignment direction on the chip in the absence of metal electrodes (data not shown), this experiment shows that the Au boundary tends to impose a local orientation on CNTs along the interfacial electric field. A more dramatic demonstration of the metal boundary effect is given by the second experiment Nano Lett., Vol. 6, No. 1, 2006

(Figure 3d-f), which is similar in its setup to the first one, except that a pair of parallel Au electrodes separated by a 500 µm wide gap is used. The metal orienting effect is clearly seen at the boundaries of the left and right electrode. Within the gap, the tube tilting angle increases almost linearly from ∼28° at the left boundary to ∼110° in the middle of the gap and finally reaches 154° at the right boundary. Note the asymmetry of the alignment pattern: the θ ) 90° occurs not at the middle of the gap, but at x ∼ 170 nm, closer to the left electrode. The alignment pattern is maintained along 57

Figure 4. (a) Normalized interaction energy (in units of kbT) as a function of distance for parameters typical of experimental conditions (see Supporting Information for details). (b) Schematic showing physical events leading to the ordered adsorption of DNACNTs onto the modified solid substrate.

the vertical (y) direction within the drop area. Similar patterns of nanotube alignment are also observed with gaps as narrow as 20 µm. The tilting angles at the two boundaries are found to deviate further away from the perpendicular orientation as the gap narrows. In what follows, we will present a model to explain all the aforementioned observations. At the core of the model is a proposed quasi-2D nematic phase for DNA-CNTs formed near the solution/substrate interface, an ordered solution state a few nanometers thick (Figure 4). To understand the physical mechanism by which ordered nanotube adsorption takes place, we consider first the interaction of a negatively charged nanotube with a hydrophobic surface that has a negative potential. Two components of the interaction are electrostatic repulsion and van der Waals attraction. To quantify this picture, we have analyzed the interaction of a charged DNA-CNT cylinder with a charged substrate under the framework of the DLVO surface interaction theory.19 We follow Manning’s approach to calculate the electrostatic potential energy.20,21 The van der Waals attraction energy term is estimated based on the interaction of a line with a half-space.19 The combined energy per unit length (in units of kbT, the thermal energy) can be written as E)

φ0λ Arctc exp(-kz) - 3 2kbT 3z k T

(1)

b

where φ0 is the surface potential, z is the distance of the tube from the substrate surface, 1/k is the Debye screening 58

length, λ is the linear charge density of DNA-CNT after the counterion condensation effect is taken into account, A is the Hamaker constant, rc is the nanotube radius, and tc is the graphene layer-to-layer distance. Equation 1 conforms to the general DLVO surface interaction potential, giving rise to an electrostatic energy barrier and a shallow secondary minimum under appropriate conditions.19 Figure 4a plots the interaction energy under different surface potential and other parameters that reasonably represent experimental conditions. The surface potential is an experimentally controllable variable, which is higher with a larger surface charge density at higher pH and lower with a thicker coating layer according to simple electrostatics. Figure 4a shows that both the height of the electrostatic barrier and the depth of the second potential minimum are critically dependent on the surface potential. One finds an electrostatic barrier of several kbT and a secondary minimum of 3-4 kbT at surface potential ∼65 mV. A consequence of the second minimum is that one may anticipate both (a) confinement of the nanotubes into a 2D plane located at the minimum and parallel to the surface and (b) an accumulation of nanotubes in this potential well. Confinement is expected because nanotube length is considerably longer than the width of the minimum so that any nonplanar configuration of the stiff nanotube will be extremely costly in energetic terms. We propose that the alignment we have observed is due to the formation of a nematic phase in the quasi-2D nanotube confinement region. Note that the CNT concentration we have used (∼0.1 to 1 µg/mL) for alignment is far lower than the typical concentration expected for the nematic phase formation in 3D. Indeed, polarized optical absorption measurements show that there is no bulk alignment in the CNT solution at ∼1 µg/mL concentration. Concentration buildup in the confinement region is thus critical to ensure a nematic phase formation. Since the existence of the second minimum depends strongly on surface potential (Figure 4a), the model provides a qualitative explanation for why a thicker hydrophobic layer does not lead to alignment. We view the ordered nanotube adsorption as a result of thermal hopping over the electrostatic barrier (Figure 4b), as suggested by its observed rate dependence on DNA-CNT solution concentration, pH, and ionic strength. Since the 2D nematic phase is bound by a finite electrostatic barrier, thermal excitation would lead to adsorption of aligned DNACNTs in the quasi-2D nematic phase onto the substrate. The rate of adsorption should therefore be determined by the height of the barrier. This explains why reducing surface charge density by lowering pH increases DNA-CNT deposition rate. Our model also predicts that the density of the adsorbed tubes initially increases linearly with the incubation time, and the adsorption rate is proportional to the bulk concentration. These are consistent with the experimental observations (Figure 2a-c and d-f) if we restrict ourselves to low adsorption densities. A characteristic feature of the nematic alignment is its sensitivity to boundary conditions. This is the phenomenon that enables many technological applications of 3D nematics. We have employed metal electrodes to create boundary Nano Lett., Vol. 6, No. 1, 2006

conditions for the proposed quasi-2D nematic phase of DNACNT. It is well-known that an internal electrostatic field as strong as 100 V/µm exists at a metal/solution interface, arising from the difference between the solution chemical potential and the metal work function.22 The field is confined typically within a small region (electrical double layer) of a few nanometers from the interface, attenuated to zero beyond the region.22 A CNT is highly polarizable along its axis and, therefore, can be aligned by an electrical field. On the basis of literature data,5 we estimate that 2 V/µm should be strong enough to orient a 500 nm long CNT at room temperature. This explains why, near a metal boundary, nanotubes have a tendency to align perpendicular to the interface. The observed tilting angle at the boundary is the result of the compromise between the metal-orienting effect and the elasticity of the quasi-2D nematic phase. We found that the observed alignment patterns in the metal electrode experiments can be quantitatively described by the continuum elastic theory of liquid crystals.18 Because the effect of the metal electrode is local in nature, it can be treated as a boundary condition on the bulk orientation of the proposed quasi-2D nematic phase. Away from the boundary, nanotube alignment is controlled by tube-tube and tube-substrate interactions. Even though both types of interaction are rather complex at the molecular level, they are nevertheless subject to simple phenomenological descriptions. At a distance x from the left electrode, we denote nanotube tilting angle with respect to the x axis by θ ) θ(x), and nanotube concentration by c ) c(x). Following wellestablished descriptions for 3D nematics,18 we write for our quasi-2D nematic phase the free energy G per unit volume at location x as 1 ∂θ 2 1 G ) ck1 + ck2 sin2(θ - θ0) + kbTc ln c 2 ∂x 2

( )

(2)

in which the first term is the elastic distortion energy due to the tube-tube interactions and k1 is the elastic constant; the second term is the surface anchoring energy due to the tubesubstrate interactions that prefer a particular tilting angle θ0, and k2 is a phenomenological surface tension constant. At equilibrium, the chemical potential of the system is everywhere the same µ(x) )

∂G 1 ∂θ 2 1 ) k + k2 sin2(θ - θ0) + kbT ln c ) ∂c 2 1 ∂x 2 constant (3)

( )

Simultaneously imposing the condition that the equilibrium distribution θ(x) minimizes the total free energy yields the following equation: k2 ∂2θ a sin 2(θ - θ0) ) 0 2 2k1 ∂x where Nano Lett., Vol. 6, No. 1, 2006

(4)

[

a) 1+

( )][

( )]

k1 dθ 2 k1 dθ 2 / 1kbT dx kbT dx

Equations 3 and 4 are the governing equations for the quasi2D nematic phase. For the one-electrode experiment, θ is close to θ0, and (dθ/dx)2 is small. We can approximate a by a positive constant close to unity. Equation 4 can then be solved analytically for θ close to θ0, resulting in an exponential decay in θ from its boundary condition on the electrode to its value far from the electrode, consistent with the experimental data (Figure 3b). Data fitting gives θ0 ) 48° and (k1/k2)1/2 ) 175 µm. Numerical solution of the nonlinear equation gives a nearly identical result. The observed concentration profile can also be explained by eq 3: it is lower near the electrode when both distortion and surface anchoring energies are high, and higher far away from the electrode when both distortion and surface anchoring energies are lower. For the two-electrode experiment, the gradient in angle is determined primarily by the boundary conditions and the distance between the electrodes, and (dθ/ dx)2 is at least 2 orders of magnitude larger than that in the one-electrode case. We can approximate a in eq 4 as a constant close to -1. Note that this changes the character of the differential equation. Equation 4 can now be solved numerically using the θ0 and (k1/k2)1/2 values obtained from the one-electrode experiment and the two boundary tilting angles. The obtained solution captures the experimental data well (Figure 3e), correctly reproducing the observed asymmetry in the alignment pattern: θ ) 90° region is not at the center of the gap, but rather shifted toward the left boundary. The observed asymmetry can also be understood from eq 3: since surface anchoring energy is lower to the left of the gap center than to its right, the distortion energy, and therefore |dθ/dx|, has to change oppositely, higher in the left region than the right region. In summary, the continuum elastic model for nematic crystals quantitatively describes the observed DNA-CNT alignment pattern. This result opens up the possibility of making required 2D nanotube alignment patterns through rational design of electrode geometry. Our observation of global alignment of deposited DNACNT implies that nanotubes in the quasi-2D nematic phase have long-range order. This seems to be opposite to the prevailing view of the liquid crystal field. It has been shown very early on that long-range order cannot exist in 2D nematic phase if the intermolecular potential is separable into positional and orientational parts.23 Using Monte Carlo simulation, several workers24-26 have also shown that the 2D nematic phase exhibits an orientational order that decays algebraically. In our case, orientation correlation function calculated from the measured tube alignment does not decay with distance. We suggest two possible ways to reconcile the difference between our observation and theoretical studies by others. One of the dominant intermolecular interactions in our case is the electrostatic repulsion between highly charged DNA-CNT hybrids. The Coulomb interaction between two charged rods is long-ranged as opposed to the short-ranged hard-core potential. It depends, among other things, on the orientation of each rod relative to the rod59

rod separation vector and therefore cannot be separated into positional and orientational parts. Thus, the basic assumptions used in the above-mentioned theoretical studies do not apply to our case, and the possibility of a 2D nematic phase with long-range order cannot be excluded. As a matter of fact, Tobochnik and Chester have shown that a nonseparable potential does result in long-range order.27 Alternatively, our quasi-2D nematic phase has a finite thickness (approximately nanometers) that may allow 3D interaction modes between nanotubes; thermal fluctuation induced long-range disorder may thus be effectively suppressed. Acknowledgment. We thank Salah Boussaad, Bruce A. Diner, Timothy D. Gierke, Ajit Krishnan, G. Bibiana Onoa, Vsevolod Rostovtsev, and Bryan Sauer for discussion and John Defranco, Bryan Sauer, Ellen Semke, and Dennis Walls for technical support. A.J. would like to thank Sujata Jagota and Manoj Chaudhury for useful discussions. This work comes from the Molecular Electronics Group at DuPont Central Research and Development. Supporting Information Available: Figures showing global alignment of DNA-CNTs on a C2-modified SiO2 surface and alkylsilane modification. This material is available free of charge via the Internet at http://pubs.acs.org. References (1) Avouris, P. Acc. Chem. Res. 2003, 35, 1026-1034. (2) Kim, F.; Kwan, S.; Akana, J.; Yang, P. J. Am. Chem. Soc. 2001, 2001, 4360-4361. (3) Huang, Y.; Duan, X.; Wei, Q.; Lieber, C. M. Science 2001, 291, 630-633. (4) Whang, D.; Jin, S.; Wu, Y.; Lieber, C. M. Nano Lett. 2003, 3, 12551259. (5) Zhang, Y.; Chang, A.; Cao, J.; Wang, Q.; Kim, W.; Li, Y.; Morris, N.; Yenilmez, E.; Kong, J.; Dai, H. Appl. Phys. Lett. 2001, 79, 31553157. (6) Huang, S.; Maynor, B.; Cai, X.; Liu, J. AdV. Mater. 2003, 15, 16511655.

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NL051952B

Nano Lett., Vol. 6, No. 1, 2006