A New Method to Synthesize Complicated Multibranched Carbon Nanotubes with Controlled Architecture and Composition
2006 Vol. 6, No. 2 186-192
Dacheng Wei, Yunqi Liu,* Lingchao Cao, Lei Fu, Xianglong Li, Yu Wang, Gui Yu, and Daoben Zhu* Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, People’s Republic of China Received October 2, 2005; Revised Manuscript Received November 17, 2005
ABSTRACT Here we develop a simple method by using flow fluctuation to synthesize arrays of multibranched carbon nanotubes (CNTs) that are far more complex than those previously reported. The architectures and compositions can be well controlled, thus avoiding any template or additive. A branching mechanism of fluctuation-promoted coalescence of catalyst particles is proposed. This finding will provide a hopeful approach to the goal of CNT-based integrated circuits and be valuable for applying branched junctions in nanoelectronics and producing branched junctions of other materials.
Individual carbon nanotubes (CNTs) have been investigated extensively and fabricated as various devices.1-4 The next step, toward a final goal to apply CNTs in nanoelectronics, individual CNTs with different properties will be integrated to a functional system for realistic applications such as integrated circuits. Much attention has been attracted by the assembly of CNTs, and many postassembly methods have been developed.5-8 However, these postassembly methods are usually complex and susceptible to contact problems.9 Branched junctions, which provide intramolecular connection and in situ self-assembly, can achieve integration more simply and reliably. In this strategy, different compositive CNTs, which exhibit distinct electronic properties,10,11 are connected as desired architecture by branched junctions. Until now many attempts have been carried out in inorganic nanomaterial through this strategy, and some complicated branched structures have been synthesized and investigated.12-16 However, the integration of CNTs by this ideal strategy is still hampered by absence of a general method to synthesize adequately complicated multibranched CNTs (MBCNTs) with complexity and controllability on both architecture and composition, although several methods for synthesizing branched CNTs have been reported, such as a Y-shaped template method,17,18 a chemical vapor deposition (CVD) method using sulfur,19 copper,20 or titanium21 doped catalyst, and nanowelding by high-intensity electron beams.22 * Corresponding authors. E-mail: [email protected]
10.1021/nl051955o CCC: $33.50 Published on Web 01/05/2006
© 2006 American Chemical Society
These methods are still far from the goals of CNT-based integrated circuits with respect to both controllability and complexity, and until now the composition, which can determine electronic properties of CNTs,10,11 still has no way to control each part. Moreover, most of these methods suffer from the disadvantage of introducing external templates17,18 or additives,19-21 which can make the synthesis process more complex and introduce impurities to influence the properties of CNTs. Furthermore, in addition to the need for CNTbased integrated circuits to fabricate branched junction based devices, such as rectifiers,23 switches,24 amplifiers,25 etc., the control of the architecture and composition is still desirable. In this communication, we developed a general, economical, and highly controllable method to resolve these problems and synthesize arrays of complicated MBCNTs with controlled architecture and composition. We only need to control the flux and the composition of gas flow, then the position, branch number, and the composition over each part can all be well controlled, avoiding any template or additive. MBCNTs were synthesized by pyrolysis of iron(II) phthalocyanine (FePc). Hydrogen gas (H2), which was used as carrier gas, was fluctuated by randomly turning up and down the flux at the middle of the reaction for 15 s. Parts a and b of Figure 1 show scanning electron microscopy (SEM) images of the product, indicating it is an array of CNTs with high-yield branched junctions. The yield of the branched junctions reaches above 90%, and most of branched junctions are located in a horizontal interface about 3 µm above the
Figure 2. (a) Cross-sectional SEM image of an array of “V” junctions on SiO2 substrate. The inset is the magnified image. TEM images indicate four stages in the formation of branched junctions (b-e) and the multiterminal “V” junctions (f). The arrows in (bf) indicate the junction area.
Figure 1. (a, b) Cross-sectional SEM images of arrays of branched CNTs on SiO2 substrate. The top middle-right inset of (b) is the high magnification image (branched junctions are contoured in white lines for clarity), and the right inset of (b) is the histogram showing the distribution of branched junctions along the vertical direction. (c-f) TEM images of four types of MBCNTs.
substrate (Figure 1b, right). The CNTs are multiwalled, and the diameter of CNTs increases gradually from the tip to the base. The stems adhere to the substrate with the catalyst particles on the base, while the longer close-ended branches are on the top without catalyst particles (Figure S1). Four types of MBCNTs can be found in the product, including “Y” junctions (Figure 1c), multiterminal junctions (Figure 1d), multilevel branched junctions (Figure 1e), and “V” junctions (Figure 1f). Almost all branched junctions have a similar configuration that two or more branches with similar diameters, which are oriented in nearly same direction, converge into a larger diameter stem. These results suggest that it predominantly follows the base growth mode where the carbon feedstock is supplied to the catalyst on the base Nano Lett., Vol. 6, No. 2, 2006
and CNTs lengthen with a closed tip. Around the junction area, the diameter of the stem corresponds to the aggregation of the branches, indicating the grain size of the catalyst for the stem is close to the aggregating particle size of the catalysts for the branches. In this method, it is not possible that the formation of branched junctions are induced by splitting of catalyst particles, because catalysts are absent at the junction area or the tips of branches but are detected at the base of thicker stems which grow after the formation of branches. So the branched junction should come from another route where neighbor catalyst particles coalesce on the base. To confirm this new mechanism, we suddenly ended the reaction right after flow fluctuation and examined the product by SEM and transmission electron microscopy (TEM). The SEM image (Figure 2a) reveals that many catalyst particles coalesced on the substrate, causing the formation of “V” junctions, while in the TEM images we can find that there are four stages in the branching process, as shown sequentially in Figure 2b-e, including the collision of catalyst particles, the coalescence of catalyst particles, the formation of branched junctions, and the growth of the stems. Moreover, the coalescence of three or more particles, which can cause the formation of multiterminal branched junctions, can also be detected from the product (Figure 2f). The flow fluctuation plays a pivotal role in the branching process. In a steady flow of 40 sccm H2 without flow fluctuation, a vertical array of CNTs without branched junctions can be synthesized (Figure S2); if the flux is as high as 1000 sccm, CNTs can be oriented along the gas flow in the growth;26 if gas flow fluctuates continuously in the whole reaction process, high-yield branched junctions can be detected at all the positions along the CNTs (Figure S3). In the steady flow of 40 sccm, the Reynolds number (Re) is very small, only 0.0679, so the flow field is stable and can be considered as both a laminar flow, which is parallel and 187
Scheme 1. The Formation Process of the Branched Junctionsa
a The process is sequential from A to E. At A, B, C, and E, the gas flow is steady. At D, the gas flow fluctuates.
steady, and a creeping flow, where viscous effects predominate and the inertia is negligible. And at the CNT growing place, the flow has already fully developed. So the drag force (F) to CNTs can be given by F ) CµVLCNT
where C is a constant, µ is the dynamic viscosity, V is the flow velocity, and LCNT is the length of CNTs. According to eq 1, the drag force is proportional to the flow velocity. At a flow of 40 sccm, the velocity is very low, only about 0.18 cm/s, CNTs can only be applied by a tiny, oriented, and constant force, so CNT arrays grow vertically on the substrate. When the flux increases to 1000 sccm, the drag force will be much bigger. So CNTs are applied by a large and uniform force field and oriented along the flow. However, if the flow fluctuates randomly, the stable laminar flow will be broken down and the flow field becomes disordered and randomly unsteady. This unsteady flow field can generate a randomly varying pressure filed on the surface of the products, apply CNTs and catalyst particles with large disordered forces, make them randomly vibrate, and largely promote the collision of catalyst particles (see Supporting Information). Then the collision particles spontaneously coalesce into larger particles as shown in Figure 2b-d, because the coalescent state, which has lower surface energy, is more stable, and the surface tension drives the coalescing process. So only at the moment of flow fluctuation can the branched junctions form, and the position of branched junctions can be well controlled when flow fluctuation is introduced as shown in Figure 1, Figure 2, Figure S3, and Figure S4. The branching mechanism is depicted in Scheme 1. First, small iron particles form from the pyrolysis of FePc and adhere to the substrate as the nucleation seeds for the growth of CNTs, then small diameter CNTs grow via the base growth mode from these particles (step A); as the reaction proceeds, catalyst particles gradually become larger and closer to each other by aggregation of iron atom from FePc, causing the synchronours growth of the diameter of the growing CNTs (steps B and C).27 Until this step, no branched 188
junction forms, because catalyst particles are in a relatively stable state and will not tend to automatically collide together due to the interval of neighbor particles, the obstacle of CNT walls, the adhesion of substrate, etc. A gentle force induced by steady flow cannot break the relatively stable state, because it cannot provide the driving force for the collision of catalyst to overcome the obstacle of CNT walls and the adhesion of substrate, although the interval is very small in the terminal stage of CNT growth. However, if the gas flow fluctuates randomly, the induced force makes catalyst particles vibrate randomly and collide with neighbors, then the collision particles spontaneously coalesce. “V” junctions form in this step (step D); finally, CNTs with larger diameter grow continuously from the coalescent particles as stems (step E). And because of the space barrier, branches are usually parallel to the stems at a small angle. In the branching process, the coalescence of particles is a key step which can determine the architecture of MBCNTs. If two catalyst particles coalesce, “Y” junctions form; if three or more particles coalesce, multiterminal junctions form, and if several particles coalesce sequentially, multilevel branched junctions form. The reaction stage, when gas flow fluctuates, is an important factor in the branching process, as branched junctions are inclined to form in the later stage of reaction. If gas flow fluctuates within the range of 40 sccm continually in the whole reaction, the yield of branched junctions suddenly increases from