ARTICLE pubs.acs.org/JPCC
Molecule Delivery by the Domino Effect of Carbon Nanotubes Qingzhong Xue,* Dan Xia, Cheng Lv, Nuannuan Jing, and Cuicui Ling College of Science, China University of Petroleum, Qingdao, Shandong 266555, P. R. China
bS Supporting Information ABSTRACT: Using molecular dynamics (MD) simulations, we demonstrate the domino effect of a carbon nanotube (CNT) whose diameter is less than 35 Å. Our simulations show that the domino wave will propagate backward, when the CNT has certain appropriate dimensions (length and diameter), after passing through the whole CNT for the first time, and then develops over the leftover standing up CNT ring again, until the energy is released. The optimal conditions for the backwardpropagating phenomenon are that the CNT diameter is about 22 Å and its length is around 225 325 Å. Molecules and atoms inside the CNT can be expelled by the domino wave at muzzle velocities of over 1 km/s, more than 1.5 times the muzzle velocity of an AK-47 machine gun. The domino effect is also found in SiC nanotubes for the first time. Our findings show a potential use for CNTs as the energy provider to deliver drug molecules, gas, water, and so on, and even can be used to design new concept domino-driven nanodevices, such as nanoinjectors, nanoperforators, and nanoguns.
1. INTRODUCTION Carbon nanotubes (CNTs), because of their exceptional mechanical,1 8 thermal,9 11 optical,12 and electrical properties,13 16 have become a hot area of research since their discovery in 1991,17 and they have been used for the development of devices for microelectromechanical and nanoelectromechanical system applications. The hollow morphology and specific large surface area provide CNTs an excellent opportunity to create nanopumping devices for atomic transportation. In the area of biomedical science and biotechnology, CNTs have been used as nanoscale vehicles for delivering drugs to targeted infected cells with unprecedented accuracy and efficiency18 20 and are not recognized as unfriendly intruders.21 So far, a large variety of actuation mechanicals, driven by thermal,22 optical,23 electrical,20,24 and mechanical25 27 effects, have been found to transport or nanopump atoms and molecules inside CNTs. Longhurst et al.22 described a methodology for the continuous pumping of decane through CNTs from a reservoir at 300 K, where they were heated and subsequently ejected from the hot end. Kral et al.23 used a two-beam coherent control to propose a laser-driven pump for atomic transport through CNTs. Using quantum mechanical molecular dynamics (QMMD) simulations, Dai et al.24 found that electrically neutral CNTs or fullerene balls housed in an outer CNT can be driven out at speeds of over 1 km/s, just like a “nanogun”, by applying a positive charge to the outer tube. On the contrary, a negatively charged tube can drive the molecule into oscillation inside it and can draw in a neutral molecule in the vicinity of its open end, like a “nanomanipulator”. Many applications of CNTs are closely related to their mechanical properties; therefore, the study of transporting or nanopumping atoms and molecules inside a CNT driven by mechanical r 2011 American Chemical Society
effects is of both scientific and technological significance. Because of their hollow structure, CNTs can encapsulate some guest molecules inside themselves through attractive interactions (e.g., van der Waals forces), and these guest molecules prefer to stay inside the CNTs because of the existing energy barrier. Therefore, developing techniques to drive atoms and molecules out of a CNT into a more useful form is significant.28 31 Applying torsion or compression to CNTs, Wang26 provided an alternative approach for atomic transportation. Chen et al.28 investigated the transport and ejection of a C20 molecule through a single-walled carbon nanotube (SWNT) achieved by a sustained mechanical actuation driven by two oscillating tips. Qiu et al.30 proposed that an excited vibrating CNT cantilever, driven by centrifugal forces, can act as an efficient and simple nanopump. Moreover, an interesting domino process in CNTs was found by Chang.31 However, such a domino process is applicable only to large tubes (diameter > 3.5 nm), which often take a ground-state collapsed shape32 rather than a metastable circular shape and are less common in synthesizing SWNTs by experiment. Herein, we use molecular dynamics (MD) simulations to demonstrate that the van der Waals (vdW) potential energy stored in SWNTs with diameters of less than 35 Å (>16.23 Å) can also be released by a domino process as an energy provider. The domino process observed in smaller CNTs is similar to that reported by Chang, but it also involves a novel phenomenon (the backward-propagating domino wave) that is obviously different from the regular domino wave and is reported for the first time. Received: July 25, 2011 Revised: September 2, 2011 Published: September 13, 2011 20471
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1 km/s. Also, the domino effect is found in SiC nanotubes for the first time. Our simulations suggest the possibility of transporting atoms and molecules by the domino process and designing novel domino-driven nanoelectromechanical system devices.
2. MODELING AND METHODS The typical model considered in this work is an armchair (15,15) CNT with a diameter of 20.34 Å and a length of 245.95 Å (Figure 1a). Two Pt nanoclusters with a size of 19.620 19.620 27.467 Å3 are placed in the vicinity of one end of the CNT and act as the external force to generate the axial collapse of the CNT. The carbon atoms that are axially constrained are shown in red, and they are fixed in the nanogun model. All simulations were implemented using the Materials Studio software based on the force field of the condensed-phase optimized molecular potential for atomistic simulation studies (COMPASS) to model the atomic interaction.33 COMPASS is the first ab initio force field that has been parametrized and validated using condensed-phase properties in addition to various ab initio and empirical data, and it has been shown to be applicable in describing the mechanical properties of CNTs.26,34 MD simulations were carried out in the constant-volume, constant-temperature (NVT) dynamics ensemble, and the dynamics process was conducted to allow the system to exchange heat with the environment. A Nose thermostat was employed to control the temperature and generate the correct statistical ensemble. As a temperature control, the thermodynamic temperature was kept constant by allowing the simulated system to exchange energy with a “heat bath”. All of the simulation processes (both the dynamics of the dominos and the dynamics of the nanogun model) consisted of two basis steps: (1) the loading process and (2) the transport process. As for the domino process, the whole simulations were carried out at room temperature 300 K, whereas the nanogun model was stimulated with an initial temperature of 1 K during the loading process. In the loading process, the time step was chosen to be 0.1 fs to make the simulations more reliable and accurate, whereas after the loading process had been applied, the time step was chosen to be 1 fs, which still satisfies the precision of the calculations, in all simulations to efficiently describe longer processes.
Figure 1. Snapshots of the domino effect in a (15,15) CNT. (a) Initial setup of the CNT/nanocluster system. (b) Configuration of the CNT after the two nanoclusters have clamped it. (c k) Configurations of the domino passing through the CNT (c) 5, (d) 10, (e) 14, (f) 16, (g) 21, (h) 26, (i) 28, (j) 29, and (k) 30 ps after the loading process was completed.
A nanogun driven by the domino process is also investigated, and the molecules inside the CNT can be driven out at speeds of over
3. RESULTS AND DISCUSSION Our MD simulation setup and the dynamics procedure are described in Figure 1. At first, two Pt nanoclusters with a size of 19.620 19.620 27.467 Å3 were placed in the vicinity of one end of the CNT, and then the two Pt nanoclusters and the CNT approached each other because of the attractive force. Because of the strong attractive force between the Pt nanoclusters and the CNT walls, the CNT began to deform and stretched its cross section from a circle to an oval along the radial direction of the CNT, which made the two Pt nanoclusters move inward and the distance between them continue to decrease, so that the interaction between the two Pt nanoclusters became stronger and stronger. After the center of mass distance of the two Pt nanoclusters reached about 28 Å, the two Pt nanoclusters were fixed, thus ending the loading process. The configuration of the CNT after the completion of the loading process is shown clearly in Figure 1b. At this moment, a clear collapse structure can be seen at the left end, and the major portion of the CNT still maintains a circular shape. To investigate the local structure of the CNT around the left end, the concentration profiles of the CNT are 20472
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Figure 2. Concentration profiles of a (15,15) CNT that partially collapsed after the loading process was completed along the (a) y and (b) z axes. The insets are the configurations of the only CNT along the y and z axes.
Figure 3. (a) Potential energy of the system versus simulation time. The inset is the system potential energy changes in the initial 30 ps. (b) Length of the collapsed CNT region and the velocity versus simulation time.
shown in Figure 2. Here, we give the concentration profiles along only the y and z axes, because there is little deformation along the CNT’s axial direction. From Figure 2, one can observe that the configuration of the left end of the CNT has been changed to 3.5 and 28.8 Å along the y and z axes, respectively, whereas the
major leftover part of the CNT has still maintained a circular shape (19.0 and 19.3 Å along the y and z axes, respectively). To investigate the motion of the structure after the loading process has been applied, the morphologies of the CNT for the time period t = 5 30 ps are shown in Figure 1c k. Once the 20473
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Figure 4. (a) Lengths of the collapsed regions of six CNTs with varied lengths versus time. The inset is the maximum backward-propagating length of the CNT versus the CNT length. The fitting curve is highlighted in blue. (b,c,f,g) Configurations of a CNT with a length of 491.90 Å after the nanoclusters clamped (b,c) one end and (f,g) the middle of the CNT. (d,e,h,i) Configurations of a CNT with a length of 491.90 Å when the domino propagates over the entire CNT and when the load is applied (d,e) at one end of the CNT and (h,i) in the middle of the CNT. (j) Configuration of just CNT, in one end of the CNT view, when the propagating process has completed.
CNT has been continuously squeezed to collapse by the two Pt nanoclusters, the collapsed region will transport axially over the whole CNT, as shown clearly in Figure 1c e. Chang31 compared this process to that of common dominos, which are lined up in a row and eventually fall down one by one after the first one is toppled. After t = 16 ps, the domino wave develops over the whole CNT along the longitudinal direction. Surprisingly, as time goes on, the domino wave propagates backward for a short distance after the domino wave had developed over the entire CNT, which is very different from the dominos in the larger-diameter CNTs (see the video in the Supporting Information). The domino wave that propagates backward is shown in Figure 1g. After that, the domino wave develops over the leftover part of the standing CNT ring again at
time t = 28 ps. After the CNT collapses entirely for the second time, the domino wave still backs off for a slight distance, which is less obvious than the first time it backed off. Finally, the CNT keeps a completely collapsed shape, showing strongly irreversible characteristics. To investigate the domino process of the CNT in detail, the curve of the total potential energy of the system as a function of time after the loading process was applied is plotted in Figure 3a. The inset is the amplified curve of the total potential energy of the system versus time for t = 0 30 ps. It is clear that the total potential energy of the system decreases dramatically in the initial 16 ps, because the domino wave develops over the whole CNT for the first time. Then, the total potential energy increases with the backward-propagating domino wave. It again decreases as the 20474
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Figure 5. (a) Length of the collapsed region of different-diameter CNTs versus time, (b) maximum backward-propagating length of the CNT versus CNT radius. The fitting curve is highlighted in red.
domino wave develops over the leftover standing-up CNT part. After t = 30 ps, the CNT has completely collapsed, showing strongly irreversible characteristics. Accordingly, the total potential energy of the system maintains a relatively constant value, even as the simulation time reaches 500 ps. Figure 3b shows the variation in the length of the collapsed CNT region with the simulation time and change in the velocity during the entire domino wave propagation. In the initial 16 ps, there are three distinct regimes on the curve of the collapsed length of the CNT against time, just as described by Chang.31 Accordingly, the initial velocity of the CNT changing with the simulation time increases slightly because the initial speed of the first domino is not too fast. However, the collapsing speed of the carbon ring reaches 1.23 km/s after 6 ps and increases slightly to 1.97 km/s at t = 13 ps. Once the domino wavefront approaches the right end of the CNT, the collapsing speed of the CNT decreases slightly because of the edge effect. When the domino wave propagates backward, the length of the collapsed CNT ring decreases dramatically, and the speed of the domino propagating backward increases linearly. Then, the domino propagation speed increases slightly forward, so that the length of the collapsed CNT ring increases step by step. When the domino wavefront approaches the right end of the CNT again, the domino propagates backward for the second time and then again propagates forward. The speed of the domino propagating varies accordingly. After t = 30 ps, the CNT collapses completely, showing strongly irreversible
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Figure 6. Axial position and velocity of the proposed nanogun versus time. (a) Configurations of the ibuprofen molecule delivered by the domino wave at 5, 12, and 17 ps. (b) Configurations of the He atom delivered by the domino wave at 4, 7, 10, and 13 ps.
characteristics, which means that the process of domino wave propagation has completed. The process of the domino propagation is a process of energy release, which allows the CNT to be an energy provider. The primary energy stored in the dominolike collapsed CNT is the vdW potential energy, which provides an attractive force to collapse the CNT. When the diameter of the CNT is over 16.23 Å, the vdW potential energy provides a driving force to collapse the CNT according to the described domino wave propagation. However, there also exists another elastic energy just as Chang described.31 For a smaller CNT whose diameter is less than 16 Å, the elastic energy is dominant, which opposes the collapse of the CNT and thus maintains its circular shape. As described above, the domino wave propagates backward after the domino wave developed over the whole CNT for the first time, which is a new phenomenon, significantly different from the results of Chang.31 Because the speed of the domino wave propagating over the whole CNT for the first time is 1.97 km/s, the energy releases in such a short time, and there is a rebound, just like with a shot: when the trigger is pulled, the bullet shoots out at a high speed, and the gun goes backward for a short distance. The second time the domino wave propagates over the leftover standing part of the CNT, the energy that is stored in it is much less than the first time, and the energy is least for the third time. Eventually, the CNT shows a completely collapsed shape. 20475
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Figure 7. (a) Snapshots of the proposed nanogun at the moment when seven molecules and atoms are just leaving the “chamber”. (b) Axial velocities of the seven molecules and atoms delivered by the domino wave as functions of time.
The domino process of the CNTs with the same diameter and different lengths was also investigated. Six different CNTs, with lengths of 122.98, 196.76, 245.95, 259.14, 368.93, and 442.71 Å, were individually placed between the two Pt nanoclusters. After the loading process had been applied, the domino wave started to pass through each entire CNT. Figure 4a shows the length of the collapsed region of the CNT as a function of simulation time. The inset is the maximum backward-propagating length of the CNT versus the CNT length. The fitting curve is highlighted in blue. From Figure 4a, one can observe that the domino wave propagates backward slightly when the CNT length is less than 200 Å, whereas it propagates backward obviously when the length of the CNT is between 200 and 300 Å. A further increase in the CNT length results in the disappearance of the phenomenon of a backward-propagating domino wave. Therefore, from Figure 4a, one can conclude that the optimal condition for the backward-propagating domino wave in a CNT is that the CNT length reaches an appropriate value (about from 225 to 325 Å). The collapse process of a very long CNT (length = 491.90 Å) was also studied. The configurations of the CNT after the loading process had been applied are shown in Figure 4b,c. The major difference in this long CNT’s collapse initiated by an external force is that the domino wave starts to propagate during the loading process and the collapse direction is orthogonal to the one that described above. This is because the initial deformation of the CNT propagates in the long CNT, whereas the deformation dies out soon in the relatively shorter CNT, which causes the domino wave to propagate before the loading process has completed. Parts b, d, f, and h of Figure 4 are side views, whereas parts c, e, g, and i of Figure 4 are top views. The configurations of the domino wave passing through the entire CNT are shown in Figure 4d,e. It is obvious that the collapse direction of the CNT ring is orthogonal to that described in shorter CNTs and that the CNT eventually forms a completely collapsed shape, similar to
double-layer graphene. The morphology of the domino wave propagating in this long CNT when the two Pt nanoclusters are placed beside the middle of the lateral CNT wall are shown in Figure 4f i. Parts f and g of Figure 4 are the configurations after the loading process has been applied. It is clear that the collapse has happened in both orthogonal directions during the loading process. A few picoseconds later, the domino wave passes through the whole CNT and eventually forms the structures shown in Figure 4h,i. Each part of the CNT that is beside the nanoclusters forms two orthogonal collapsed sections, and the lengths of the collapsed two sections are essentially the same. This might be because the forces formed by the two sections are nearly equal and eventually form the stable structures shown in Figure 4h, i. The structure of just the CNT, at one end of the CNT view, is shown in Figure 4j. One can observe a clear crisscross formed by the two sections of the collapsed CNT. Also, the effect of the CNT diameter on the domino wave was investigated. Figure 5a shows the length of the collapsed region as a function of simulation time. From Figure 5a, one can observe that the backward-propagating phenomenon is slight in (13,13) and (14,14) CNTs, whereas it is obvious in (15,15), (18,18), and (20,20) CNTs. When the diameter of the CNT is over 30 Å [(24,24) and (25,25) CNTS], the backward-propagation phenomenon can almost be neglected. Figure 5b shows the maximum backward-propagating length of the CNT versus the CNT radius. It is obvious that the maximum backward-propagating length of the CNT increases with increasing CNT radius before the CNT radius reaches ∼22 Å. A further increase in the CNT radius results in a dramatic decrease in the maximum backwardpropagating length of the CNT, and finally, the backwardpropagating process can almost be neglected when the CNT diameter is over 30 Å. The results in Figure 5 might be because the vdW potential energy is dominant in the competition between the vdW potential energy and the elastic energy when the 20476
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Figure 8. Snapshots of a large number of water molecules expelled from the CNT by the domino wave: (a) partial, (b) total.
Figure 9. Snapshots of the domino effect in a (12,12) SiC nanotube. (a) Initial setup of the nanotube/nanocluster system. (b) Configuration of the SiC nanotube after two nanoclusters have clamped the SiC nanotube. (c f) Configurations of the domino passing through the SiC nanotube at (c) 2, (d) 7, (e) 11, and (f) 16 ps after the loading process was completed.
CNT diameter is over 16.23 Å. However, the vdW potential energy is slightly stronger than the elastic energy in the smaller CNTs, such as the (13,13) and (14,14) CNTs. Therefore, the energy rebounding by the domino wave propagation can make the collapsed CNT wall stand up for only a short distance. Upon a further increase in the CNT radius, the vdW potential energy is
relatively much stronger than the elastic energy, as in (15,15) CNTs. As a result, the energy rebounded by the domino wave propagation can make the collapsed CNT wall stand up for a relatively longer distance. When the diameter of the CNT is over ∼22 Å, the vdW potential energy is much stronger than the elastic energy, so the vdW potential energy can bond the collapsed CNT walls more strongly. Therefore, the energy rebounded by the backward-propagating phenomenon that makes the collapsed CNT wall stand up again is less and less obvious. Eventually, the backward-propagating domino wave phenomenon can be neglected or even will not exist in relatively larger-diameter CNTs (diameter > 30 Å). The vdW potential energy stored up in the CNT can be the driving force to initiate the domino wave propagation; that is, the vdW energy can be partly converted into kinetic energy, which can be used to drive this energy source to a more useful form. The common approach to making this energy into a useful form is to make nanodevices that can be used to deliver atoms and molecules to a target place, such as delivering drug molecules to targeted infected cells with unprecedented accuracy and efficiency. Figure 6 shows a prototype design of a domino-driven nanogun. Once the two Pt nanoclusters (acting as the trigger) clamp the CNT, the collapse develops over the whole CNT, which squeezes the inside molecules out at a high speed in a few picoseconds. The axial position and velocity curves of an ibuprofen molecule and a He atom expelled from a CNT by a domino wave are plotted in Figure 6. The insets of Figure 6a are the configurations of the ibuprofen molecule shot out by the nanogun at 5, 12, and 17 ps, whereas the insets of Figure 6b are the configurations of the He atom shot out by the nanogun in 4, 7, 10, and 13 ps after the trigger is pulled. The fixed carbon atoms are shown in red. The curves of the axial position and axial velocity of the ibuprofen molecule versus time, shown in Figure 6a, are very similar to the curves of the collapsed CNT length and velocity varying with the simulation time when the domino wave propagates along the whole CNT for the first time. This might be because the kinetic energy of the domino wave is completely converted into both the kinetic energy of the ibuprofen molecule and the kinetic energy of the collapsed CNT; thus, they have the same velocity just like the impact. The insets in Figure 6a show that the ibuprofen molecule is very close to the collapsed region of the CNT and that they move together, which also confirms this conclusion. However, the process of the He atom expelled by the domino wave is slightly different from that of the ibuprofen molecule. The axial position and axial velocity of the He atom as functions of simulation time are illustrated in Figure 6b. It is clear that the delivery process has three distinct phases. The first phase is the initial propagation of 20477
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Figure 10. Snapshots of a C60 molecule delivered by the domino effect of a (12, 12) SiC nanotube. (a) Initial setup of the proposed nanogun. (b) Configuration of the SiC nanotube after the loading process applied at (A) 1 and (B) 300 K. Configurations of a C60 molecule delivered by the domino effect at (A) (c) 4, (d) 6, (e) 10, and (f) 28 ps and (B) (c) 3, (d) 7, (e) 11, and (f) 15 ps.
the domino wave, which is complicated. The second phase is the domino wave “impact” the He atom, thus giving the He atom a constant velocity that is higher than that of the domino wave of the CNT while the domino wave accelerates as usual. At t = 7 ps, the domino wave catches up with the He atom and impacts it again, thus accelerating the He atom to a higher constant velocity while the domino wave accelerates continuously. When the domino front approaches the right end of the CNT, the domino wave decelerates because of the edge effect, whereas the He atom moves on with the same velocity as previously. Therefore, when the He atom is expelled out of the CNT, the domino wave has not already passed through the CNT. This is the third phase. Seven kinds of molecules and atoms were selected to research the delivery characteristics of the CNT. They are C60, cinnamamide, ibuprofen, topotecan, CH4, He, and H2O. The configurations of these seven molecules and atoms just squeezed out of the CNT are shown in Figure 7a. From Figure 7a, one can observe that four kinds of molecules (C60, cinnamamide, ibuprofen, topotecan) are just expelled out of the CNT, whereas the other three are expelled out of the CNT for a distance. The axial velocities of the seven molecules and atoms versus time are shown in Figure 7b. The muzzle velocities of the seven molecules and atoms are 1.165, 1.146, 1.228, 1.248, 1.488, 2.326, and 1.537 km/s
for C60, cinnamamide, ibuprofen, topotecan, CH4, He, and H2O, respectively. Combining Figure 7a with Figure 7b, one can conclude that the delivery mechanism of the first four kinds of molecules (C60, cinnamamide, ibuprofen, topotecan) is like that in Figure 6a, whereas the delivery mechanism of the other three is like that in Figure 6b. This is why the muzzle velocities of the first four are lower than those of the other three. Moreover, the domino wave can deliver not only a single molecule or atom but also a large number of molecules. Figure 8 shows the configurations of a large number of H2O molecules expelled from a CNT by the domino wave. These H2O molecules are squeezed out of the CNT in part, as shown in Figure 8a, whereas all of these H2O molecules are expelled from the CNT in Figure 8b. The same occurs for the six other molecules and atoms, with the only difference being the number of the molecules that can be delivered by the domino wave of the CNT. In addition, we found the domino effect in a SiC nanotube for the first time. Here, the SiC nanotube that we chose has a diameter of 19.98 Å and a length of 227.01 Å, which are similar to the parameters of the CNTs that we discussed above. Moreover, the Pt nanoclusters are the same as for the CNT system. Figure 9 shows the dynamic process of the domino wave propagating over the SiC nanotube. The major difference between the domino 20478
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The Journal of Physical Chemistry C wave of the CNT and that of the SiC nanotube is that the atoms of the SiC nanotube far beyond the nanoclusters semicollapsed whereas the atoms of the CNT beyond the nanoclusters deformed little. A possible reason for this difference is that the tenacity of the SiC nanotube is weaker than that of the CNT. From Figure 9, one can observe that, in the initial 2 ps, the domino wave of the SiC nanotube passed a distinct distance that was much greater than that in the CNT. When the domino wavefront approaches the other end of the SiC nanotube, the propagation speed decreases because of the edge effect. After 16 ps later, the domino wave has already passed through the whole SiC nanotube, and the nanotube collapses completely with irreversible characteristics. Here, we did not find a backwardpropagating phenomenon in this SiC nanotube, because the tenacity of the SiC nanotube is very weak, not as strong as that of CNTs. That is, the vdW potential energy is much stronger than the elastic energy in SiC nanotubes, which is similar to the case when the CNT diameter is over 30 Å. The molecules delivered by the domino wave of the SiC nanotube were also investigated. Figure 10 shows the dynamics process of a C60 molecule delivered by the domino wave of a SiC nanotube. Because the tenacity of the SiC nanotube at low temperature (1 K) is much weaker than that at room temperature (300 K), the configuration of the SiC after the loading process has been applied, as shown in Figure 10Ab, is almost semicollapsed. At t = 4 ps after the loading process, the domino wave impacts the C60 molecule and pushes it along the axis direction. Soon, the SiC nanotube forms another domino wave at the other end and passes through the SiC nanotube in the reverse direction at t = 6 ps after the loading process was applied, because of the initial semicollapsed shape. At t = 10 ps, these two domino wave meet in the middle of the chamber, thus trapping the C60 molecule in the SiC chamber. As time goes on, the domino wave from the other end of the nanotube pushes C60 back and eventually stay in the rear of the clamped region of the nanoclusters. When the loading process is applied at room temperature (300 K), the configuration of the CNT after the loading process has been applied is partly collapsed around the nanoclusters, as shown in Figure 10Bb, because of the stronger tenacity. However, the C60 is delivered for a short distance. At t = 3 ps, the domino wave impacts the C60 molecule and pushes it along the axis direction, and the next delivery process is similar to that in the CNT. Thus, although we found the domino effect in the SiC nanotube, the SiC nanotube is not a good candidate for delivering molecules as a transport vehicle, because of its weak tenacity.
4. CONCLUSIONS In summary, using MD simulations, we investigated the distinctive domino effect in the CNT. We found that, when the CNT diameter is larger than 16.23 Å, the domino wave can propagate over the whole CNT. More interesting, the domino wave will propagate backward when the CNT diameter is about 22 Å and the length of the CNT is around 225 325 Å. Because the vdW potential energy can convert into kinetic energy, a prototype of a nanogun expelling seven molecules and atoms from the CNT was also demonstrated. All of the muzzle velocities of the seven molecules and atoms were found to be over 1 km/s, which is more than 1.5 times the muzzle velocity of an AK-47 machine gun. Moreover, we found the domino effect in SiC nanotubes for the first time; nevertheless, the particle delivery characteristics are not as good as for CNTs. Our proposed domino effect
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provides a useful mechanism for using CNTs as an energy source to deliver drug molecules, gas, water, and so on, and even provides opportunities to design future domino-driven nanodevices, such as nanoinjectors, nanoperforators, and nanoguns.
’ ASSOCIATED CONTENT
bS
Supporting Information. MD simulation showing details of the domino passing through the whole CNT after the loading process is applied. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT This work was supported by the Cultivation Fund of the Key Scientific and Technical Innovation Project, Ministry of Education of China (708061); Natural Science Foundation of China (10974258); and Program for New Century Excellent Talents in University (NCET-08-0844). ’ REFERENCES (1) Zheng, Q. B.; Xue, Q. Z.; Yan, K. Y.; Hao, L. Z.; Li, Q.; Gao, X. L. J. Phys. Chem. C 2007, 111, 4628–4635. (2) Wagner, H. D.; Lourie, O.; Feldman, Y.; Tenne, R. Appl. Phys. Lett. 1998, 72, 188–190. (3) Andrews, R.; Jacques, D.; Rao, A. M.; Rantell, T.; Derbyshire, F.; Chen, Y.; Chen, J.; Haddon, R. C. Appl. Phys. Lett. 1999, 75, 1329–1331. (4) Ajayan, P. M.; Schadler, L. S.; Giannaris, C.; Rubio, A. Adv. Mater. 2000, 12, 750–753. (5) Qian, D.; Dickey, E. C.; Andrews, R.; Rantell, T. Appl. Phys. Lett. 2000, 76, 2868–2870. (6) Cadek, M.; Coleman, J. N.; Barron, V.; Hedicke, K.; Blau, W. J. Appl. Phys. Lett. 2002, 81, 5123–5125. (7) Dalton, A. B.; Collins, S.; Mu~ noz, E.; Razal, J. M.; Ebron, V. H.; Ferraris, J. P.; Coleman, J. N.; Kim, B. G.; Baughman, R. H. Nature 2003, 423, 703–703. (8) Cadek, M.; Coleman, J. N.; Ryan, K. P.; Nicolosi, V.; Bister, G.; Fonseca, A.; Nagy, J. B.; Szostak, K.; Beguin, F.; Blau, W. J. Nano Lett. 2004, 4, 353–356. (9) Clancy, T. C.; Gates, T. S. Polymer 2006, 47, 5990–5996. (10) Wei, C. Y.; Srivastava, D.; Cho, K. Nano Lett. 2002, 2, 647–650. (11) Pham, J. Q.; Mitchell, C. A.; Bahr, J. L.; Tour, J. M.; Krishanamoorti, R.; Green, P. F. J. Polym. Sci. B: Polym. Phys. 2003, 41, 3339–3345. (12) Kymakis, E.; Amaratunga, G. A. J. Appl. Phys. Lett. 2002, 80, 112–114. (13) Yan, K. Y.; Xue, Q. Z.; Zheng, Q. B.; Hao, L. Z. Nanotechnology 2007, 18, 255705. (14) Kim, B.; Lee, J.; Yu, I. J. Appl. Phys. 2003, 94, 6724–6728. (15) Ramasubramaniam, R.; Chen, J.; Liu, H. Y. Appl. Phys. Lett. 2003, 83, 2928–2930. (16) Sandler, J. K. W.; Kirk, J. E.; Kinloch, I. A.; Shaffer, M. S. P.; Windle, A. H. Polymer 2003, 44, 5893–5899. (17) Iijima, S. Nature 1991, 354, 56–58. (18) Martin, C. R.; Kohli, P. Nat. Rev. Drug Discov. 2003, 2, 29–37. (19) Kam, N. W. S.; Dai, H. J. J. Am. Chem. Soc. 2005, 127, 6021– 6026. (20) Svensson, K.; Olin, H.; Olsson, E. Phys. Rev. Lett. 2004, 93, 145901. (21) Pantarotto, D.; Briand, J.-P.; Prato, M.; Bianco, A. Chem. Commun. 2004, 1, 16–17. 20479
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