Atomic Scale Simulation of the Contact Behaviour and Mechanism of

Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Atomic-Scale Simulation of the Contact Behavior and Mechanism of the SWNT−AgNW Heterostructure Jianlei Cui,*,†,‡,§,∥ Jianwei Zhang,†,‡ Xuewen Wang,†,§,⊥ Barayavuga Theogene,† Wenjun Wang,† Hironori Tohmyoh,*,∥ Xiaoqiao He,⊥ and Xuesong Mei*,† †

State Key Laboratory for Manufacturing Systems Engineering, Xi’an Jiaotong University, Xi’an 710049, P. R. China State Key Laboratory of Fluid Power and Mechatronic Systems, Zhejiang University, Hangzhou 310027, P. R. China § State Key Laboratory of Transient Optics and Photonics, Xi’an Institute of Optics and Precision Mechanics of Chinese Academy of Sciences, Xi’an 710119, P. R. China ∥ Department of Finemechanics, Tohoku University, Aoba 6-6-01, Aramaki, Aoba-ku, Sendai 980-8579, Japan ⊥ Department of Architecture and Civil Engineering, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong

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‡

S Supporting Information *

ABSTRACT: We investigated the interfacial contact behavior of the side-to-side biaxial heterostructure between carbon nanotubes and silver nanowires on an atomic scale. The nanotubes can move along the nanowire periphery and keep pace with the silver nanowires, and in some cases, a collapse occurs and quickly creates a domino effect that readily forms the bilayer graphene-like structures with a face-to-face π−π stacking effect that adhere firmly to the nanowire surface. When the diameter of an armchair nanotube is very large, the bilayer graphene-like structure that has been formed can scroll onto the nanowire periphery and wrap around the nanowire to form a core/shell hybrid structure that will eventually be transformed into a double-walled carbon nanotube structure. In other circumstances that are affected by factors such as temperature and the nanotube structure, the carbon nanotube does not easily collapse; instead, it retains its intrinsic circular form. The mechanism for interfacial contact behavior reveals that the van der Waals interactions play an important role in the entire process. The effects of the interfacial contact behavior and the final atomic configuration may provide valuable theoretical guidance for designing and fabricating hybrid structures with broad potential applications, such as nanoelectronic devices and functional composite materials. conventional metals.14−19 Banhart et al.20,21 studied the controlled formation of heterojunctions between carbon nanotubes and different metal nanocrystals (Fe, Co, Ni, and FeCo) that acted as ultimate end-nanocontacts. In addition, multisegmented one-dimensional hybrid end-to-end contact structures between CNTs and metal nanowires were fabricated by Ajayan et al.22−24 on alumina templates with 60 nm nanopore diameters; the growth of the metal nanowires was achieved using electrodeposition techniques, which was followed by the growth of CNTs using chemical vapor deposition. Cui et al.25−28 theoretically and experimentally studied the heterogeneous nanojunctions between multiwalled carbon nanotubes and an Ag nanoparticle based on the nearfield enhancement effect with a laser irradiating nanoprobe tip. Lee et al.29 reported highly transparent and flexible metal NW/ CNT hybrid networks on PET substrates combined with plasmonic welding for securing ultrahigh stability in the

1. INTRODUCTION Carbon nanotubes (CNTs) and metal nanowires (NWs) are interesting new building blocks for “bottom−up” fabrication and are promising candidates for use in nanocircuits, flexible electronics devices, and CNT/NW-reinforced composite materials; they are promising due to their remarkable mechanical, electrical, and thermal properties, as well as their other unique characteristics. Functional composite structures made from CNTs and NWs have attracted significant attention and have broad application prospects in fields such as nanodevices, supercapacitors, hydrogen storage materials, conductive films, heterogeneous catalysis, wave-transmitting materials, and advanced stealth composites.1−13 The CNT/ NW composite structures that are currently in use include the typical heterostructures with end-to-end coaxial contact, endto-side contact, side-to-side biaxial contact, and crossing contact. Because the advantages of heterojunctions between CNTs and metal nanowires were demonstrated in the nanoelectromechanical system, high-power supercapacitor, and high-frequency application, it follows that effective nanocontacts can be created between CNTs and nanowires or © XXXX American Chemical Society

Received: May 31, 2019 Revised: July 17, 2019 Published: July 22, 2019 A

DOI: 10.1021/acs.jpcc.9b05181 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

The energy of the valence interactions Evalence is generally accounted for by the diagonal terms50

mechanical and electrical properties under sever bending. To date, research on the fabrication of heterostructures is still relatively lacking. To better understand the interactions on the interface between CNT and metal NW, theoretical research on the heterogeneous contact is needed to explore how essential factors affect the contact behavior and the atomic structures. Unfortunately, the interaction behavior and the evolution of the atomic configurations can hardly be observed at the atomic scale, and experimental study of these interactions and structures requires integrated complex operations of in situ nanomanipulation, nanojoining, and nanotesting.30−36 Additionally, it is difficult to accurately calculate the relevant parameters by theoretical and numerical formulas based on nanometer-scale effects of nanotubes or nanowires with exceptionally high surface-to-volume ratios. However, molecular dynamics (MD) simulations are a powerful tool for demonstrating the dynamic evolution of atomic-scale behavior. Yan et al.37 studied the interactions in CNT/Cu composite nanowires via forced-field-based MD simulations and identified their potential for use in altering the properties of the pristine tube, since it is a natural layer that can protect against oxidation and shape fragmentation. Furthermore, Chen et al.38 investigated the interactions between carbon nanorings and Al nanowires with MD simulations and demonstrated that the nanowire can activate, guide, and stabilize the self-assembly of the carbon nanotubes to form a double-deck helix. To date, little is known about the interfacial contact behavior, mechanisms, and atomic configurations and further exploration is warranted. Here, we perform MD simulations on side-to-side biaxial heterostructures between single-walled carbon nanotubes (SWNT) and silver nanowires (AgNW) to reveal contact behavior and mechanisms on the atomic scale. Moreover, we investigate the influence of many essential factors on the SWNT−AgNW composite configurations. This work not only is vital for understanding the interesting interaction processes but will also help provide meaningful guidance for the design, fabrication, and application of novel SWNT−AgNW heterostructures.

Evalence = E bond + Eangle + Etorsion + Eoop + E UB =

θ

+

∑ [H2(θ − θ0)2 + H3(θ − θ0)3 θ

+ H4(θ − θ0)4 ] +

∑ [V1[1 − cos(φ)] + V2[1 − cos(2φ)] φ

+ V3[1 − cos(3φ)]] +

∑ Kxx 2 + EUB x

(2)

where Ebond, Eangle, and Etorsion represent the bond stretching, valence angle bending, and dihedral angle torsion, respectively. Eoop is the inversion out-of-plane interactions (oop) terms, which are part of the force fields for covalent systems. EUB gives the Urey−Bradley (UB) term, which may be used to account for the interactions between the atom pairs involved in 1−3 configurations (i.e., atoms bound to a common atom). For a higher accuracy, the force field is achieved by including cross-terms to account for such factors as bond or angle distortions caused by nearby atoms.51 Ecross ‐ term = E bond ‐ bond + Eangle ‐ angle + E bond ‐ angle + Eend ‐ bond ‐ torsion + Emiddle ‐ bond ‐ torsion + Eangle ‐ torsion + Eangle ‐ angle ‐ torsion =

∑ ∑ Fbb′(b − b0)(b′ − b0′) b

+

b′

∑ ∑ Fθθ′(θ − θ0)(θ′ − θ0′) θ

+ +

θ′

∑ ∑ Fbθ(b − b0)(θ − θ0) b

θ

∑ ∑ (b − b0)(V1cos φ + V2cos 2φ b

2. COMPUTATIONAL METHODS MD simulations were carried out using the Discover module of Materials Studio software, which was developed by Accelrys Software Inc. The interatomic force field determines the work load and accuracy, so the interactions were implemented by the powerful force field of Condensed-phase Optimized Molecular Potentials for Atomistic Simulation Studies (COMPASS).25,37,39−41 It is a parameterized, tested, and validated first ab initio force field, and most parameters were derived based on ab initio data. The parameterization procedure can be divided into two phases: ab initio parameterization and empirical optimization. The COMPASS force field can enable an accurate and simultaneous prediction of the condensed-phase properties (equation of state, cohesive energies, etc.) of metal and nonmetal materials, such as Ag and C.42−49 The potential energy Epotential of a system can be expressed as the sum of the valence Evalence, the cross-term Ecross‑term, and the nonbond interactions Enonbond, with the following equation Epotential = Evalence + Ecross ‐ term + Enonbond

∑ [K 2(b − b0)2 + K3(b − b0)3 + K4(b − b0)4 ]

φ

+ V3cos 3φ) +

∑ ∑ (b′ − b0′) b′

φ

(V1 cos φ + V2 cos 2φ + V3 cos 3φ) +

∑ ∑ (θ − θ0)(V1 cos φ + V2 cos 2φ θ

φ

+ V3 cos 3φ) +

∑ ∑ ∑ Kφθθ′ cos φ(θ − θ0)(θ′ − θ0′) φ

θ

θ′

(3)

The different energy terms Ebond‑bond, Eangle‑angle, Ebond‑angle, Eend‑bond‑torsion, Emiddle‑bond‑torsion, Eangle‑torsion, and Eangle‑angle‑torsion, respectively, give the stretch−stretch interaction between two adjacent bonds, the bend−bend interaction between two valence angles, the stretch−bend interaction between a twobond angle and one of the bonds, the stretch−torsion interaction between a dihedral angle and one of the end bonds, the stretch−torsion interaction between a dihedral angle and the middle bond, the bend−torsion interaction

(1) B


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Figure 1. (a) Atomic model/initial configuration and snapshots of the instantaneous side-to-side biaxial heterogeneous contact configurations at (b) 100 ps, (c) 300 ps, (d) 330 ps, (e) 500 ps, and (f) 1ns between unsaturated (15, 15) SWNT and AgNW at the temperature of 500 K.

the integration method for the simulation computations. To extract stable and accurate results, the total simulation time was set to 1 ns, with 500 000 time steps, the trajectories of all atoms were stored, and the frame was output every 5000 steps. For an analysis of different interfacial contact behaviors, AgNW with 8.0 nm length and 2.0 nm diameter is selected and fixed in MD simulations and the structure and size characteristics of SWNT could be changed to eliminate different size effects based on changeable AgNW. In addition, as we know, CNTs can be produced with different methods and are usually sealed at both ends. When CNTs are used to fabricate nanocircuits, they are usually cut into different building blocks with different sizes having unsealed ends. Sometimes, CNTs can be also treated with saturation at the unsealed ends or functional modification at the outer wall of the nanotube based on the functionalization groups (−H, −OH, −CH3, −NH2) in the applications of nanodevices, biochemistry, etc. So, unsealed SWNTs are chosen in MD simulations.

between a dihedral angle and one of the valence angles, and the bend−bend−torsion interaction between a dihedral angle and its two valence angles. In addition, the nonbond energy Enonbond, containing the L-J 9-6 potential function for the van der Waals interaction term EvdW, a Coulombic term for the electrostatic interaction, ECoulomb, and the hydrogen bond energy, EH‑bond, accounts for the interactions between the nonbond atoms.52 Enon ‐ bond = EvdW + ECoulomb ÄÅ ÅÅ i 0 y9 ÅÅ j rij z = ∑ εijÅÅÅÅ2jjjj zzzz − ÅÅ j rij z i>j ÅÅÇ k {

+ E H ‐ bond É 6Ñ ij rij0 yz ÑÑÑÑ 3jjjj zzzz ÑÑÑÑ + j rij z ÑÑ k { ÑÑÖ

∑ i>j

qiqj rij

+ E H ‐ bond (4)

where

ij (ri0)6 + (r j0)6 yz j zz = jjj zz j z 2 k {

1/6

rij0

3. RESULTS AND DISCUSSION 3.1. Behavioral Evolution of the Side-to-Side Biaxial SWNT−AgNW Heterostructure. Figure 1a shows a typical atomic model for the side-to-side biaxial heterostructure between an unmodified SWNT and a AgNW with a contact length corresponding to actual common situations. The selection was made for an armchair SWNT with a chiral index of (15, 15), a length of 8.116 nm, and a diameter of 2.034 nm for the MD simulation to minimize the impact of multiple complex factors on the contact behavior. Additionally, Figure 1 shows the images of the instantaneous side-to-side biaxial heterogeneous contact configurations at different times at a relatively low temperature of 500 K. Based on the evolution of the contact configurations shown in Figure 1, the geometric central position of AgNW is minimally changed, but the SWNT can move along the nanowire peripheral surface as the contact length increases. At 100 ps, the nanotube maintains a relatively good intrinsic circular form without an obvious deformation. With time increasing to 300 ps, the nanotube continues to move rapidly along the periphery of the nanowire to reach the AgNW and keep pace with it. The atomic morphology of the SWNT exhibits an obvious deformation in its right side, while the left side remains unchanged. Next, the deformed structure begins to collapse until a complete collapse occurs at 330 ps. However, the structure on the left part of the SWNT maintains a relatively good intrinsic circular form during this process. At 500 ps, the SWNT had completely collapsed. Figure 2 depicts

(5)

In the simulations, taking nanometer-size effects into account, the x, y, and z directions were imposed by the nonperiodic boundary conditions and the interatomic force field of COMPASS was selected. In the actual system, the molecular chain exists in a stable conformation; that is, it is at a lower energy conformation. For structure optimization before MD simulations, the initial model was first simulated with molecular mechanics (MM) with the “Smart Minimizer” method and the convergence criteria and maximum interactions were set to 1000 kcal/mol/Å and 5000, respectively. Then, the optimized model was simulated by the MD method under the constant NVT (N is the number of atoms in the system, V gives the volume, T represents the temperature) ensemble. Also, an Andersen thermostat was chosen to control the system temperature. In subsequent MD simulations, the van der Waals (vdW) interactions were calculated by the summation method of the atoms based on a cutoff distance of 9.50 Å, a spline width of 1.00 Å, and a buffer width of 0.50 Å. In addition, the initial velocities of all atoms were fit to a Maxwell−Boltzmann distribution. However, the Maxwell−Boltzmann distribution applies to the statistics of a large number of atoms with greater fluctuation results. To reduce the energy fluctuation, a larger simulation time step was employed, but it increases the time for the stabilization of the energy. Therefore, to balance these two effects, a time step of 2 fs was used in the simulation and Velocity Verlet was chosen as C


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Figure 2. Snapshots of the collapse behavior of SWNT with atomic contact configurations at different times of (a) 320 ps, (b) 330 ps, (c) 340 ps, and (d) 350 ps between the unsaturated (15, 15) SWNT and AgNW at the temperature of 500 K.

Figure 3. Different interaction energies versus the entire and isolated systems of (15, 15) SWNT−AgNW heterostructures at the temperature of 500 K including (a) the stable Epotential, Evalence, Ecross‑term, and Enonbond of the final atomic configuration and (b) the corresponding energy differences (ΔEpotential, ΔEvalence, ΔEcross‑term, ΔEnonbond) between the final atomic configuration and the initial atomic configuration versus different systems of the SWNT−AgNW, SWNT, and AgNW.

the integral collapse behavior of the SWNT in this short time period with a series of atomic contact configurations at 10 ps intervals during the nanotube collapse process. At 320 ps, the right side of the nanotube, as a precollapse structure, shows obvious deformation that is similar to that in Figure 1c. After 10 ps, the complete collapse deformation appears on the right side of the nanotube to form a bilayer graphene-like structure (∼0.35 nm distance between the upper and lower layers of collapsed nanotube), which tightly adheres to the nanowire periphery. As time progresses, the deformed portion successively collapses within a remarkably short time, which appears to trigger a complex domino effect. When the time reaches 350 ps, the hollow cylindrical nanotube completely collapses into a bilayer graphene structure and eventually adheres to the nanowire peripheral surface, which is basically equivalent to the final contact configuration of the SWNT− AgNW composite structure at 1 ns. Furthermore, during the heterogeneous interfacial contact process shown in Figure 1, the length of SWNT remains essentially unchanged, but the length and configuration of the nanowire significantly change. Before the SWNT moves along the nanowire peripheral surface and reaches the AgNW, the nanowire length gradually becomes shorter during the time span of 0−330 ps. The reduction in the nanowire length occurs because the Ag atoms at the tip of the right end of the nanotube are subjected to a strong attraction from the C atoms dangling from the right tip of the SWNT. Inversely, no significant atomic interaction exists between the C atoms dangling from the left tip of the SWNT and Ag atoms from the left end of the AgNW because the distance between them is too great. There is also a large space between the C atoms dangling from the left tip of the SWNT and the Ag atoms dangling from the left end of the AgNW until the nanotube catches up with the nanowire and ultimately keeps pace with it. Then, the Ag atoms on the left part of the nanowire experience strong attractions from the dangling C atoms of the nanotube

tip, which causes the nanowire to gradually lengthen and then reach the relatively stable state of the contact figuration shown in Figure 1f. 3.2. Mechanism for Behavior Evolution of the Sideto-Side Biaxial SWNT−AgNW Heterostructure. To elucidate the mechanism for the above-mentioned behavior of the SWNT−AgNW composite heterostructure, the simulation temperature was kept constant at a value of 500 K during the interaction process (Figure 1), which allowed the kinetic energy of the simulation systems to be ignored, leaving only the influence of the potential energy of the atomic configuration and its components to be considered. To understand the primary major energy components affecting the structures of the different systems, the stable Epotential, Evalence, Ecross‑term, and Enonbond values of the final atomic configuration versus the entire and isolated system of the unsaturated (15, 15) SWNT−AgNW heterostructures at the temperature of 500 K were evaluated, as shown in Figure 3a. The cross-term energy Ecross‑term percentage is negligible and is not a critical factor for any of the systems. The valence energy, Evalence, does not exist in the isolated AgNW system, and the SWNT system has very little nonbond interaction energy, Enonbond. Therefore, the valence energy, Evalence, and the nonbond interaction energy, Enonbond, which are the sole determining factors for the potential energy of the detached SWNT and the AgNW system, respectively, are the decisive components of the SWNT−AgNW integrated system. Additionally, because the Coulombic energy, ECoulomb, and the hydrogen bond energy, EH‑bond, have a value of zero in the MD simulation, the nonbond interaction energy, Enonbond, depends on the vdW interaction energy, EvdW, according to eq 4. Although the main components of the system energy are now clear, the decision of the atomic configuration does not depend on the final energy, and it is difficult to identify the critical factors that lead to the structural changes. Figure 3b depicts the energy differences between the final atomic configuration D


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SWNT collapsing and adhering to the AgNW periphery, as shown in Figure 2. The nonbond energy that is influenced by the vdW interaction energy also causes a change in the contact composite structures when the SWNT is moving along the AgNW periphery. The collapse of the nanotube begins at 320 ps and ends at 350 ps, with a corresponding deformation energy of 1237.6 kcal/mol that is based on the decreasing energy before 320 ps. Minor adjustments to the final atomic configuration ensues after 350 ps for energy minimization of the system. Consequently, the vdW interaction plays an important role in the interfacial contact behavior between SWNT and AgNW. To reflect the adhesion intensity of the SWNT−AgNW system, the binding energy, Ebinding, was calculated as the negative value of the interaction energy, Einteraction, which is the potential energy difference between the integrated system and the sum of all the isolated subsystems, according to the following equation.

and the initial atomic configuration versus the different systems, according to Figure 3a. Comparison of the energy difference between the integrated SWNT−AgNW system and the isolated systems shows that, although the valence energy and the cross-term energy have changed, they are not the main factors affecting the contact motion behavior and the final atomic configuration that is shown in Figure 1. The nonbond energy is equal to the vdW interaction energy, EvdW, which demonstrates that there is an essential change from a positive value in each isolated system to a negative value in integrated system. Thus, the vdW interaction is the critical factor in the atomic configuration evolution and the structural distortion of the SWNT shown in Figure 1 causes the changes in its valence energy and its detailed intramolecular energy in the Supporting Information. Further verification is seen in Figure 4, which

Einteration = ΔE = ESWNT − AgNW − ESWNT − EAgNW

(6)

Thus, the binding energy can reach the value of 1883.23 kcal/ mol, which allows the SWNT and AgNW to adopt a compact conformation, as shown in Figure 1f. 3.3. Interaction Model for the Interfacial Contact Behavior of the Side-to-Side Biaxial SWNT−AgNW Heterostructure. For further analysis of the interfacial contact behavior described above, the interaction model between the unsaturated SWNT and AgNW with side-toside biaxial heterogeneous nanojunctions (Figure 5) was examined. The SWNT behavior consists of three important steps. Stage I: In the initial contact configuration, there is a relatively large gap between the SWNT and the AgNW in the x−y plane, but the Ag atoms on the lower surface of the AgNW (marked in green) and the C atoms on the upper surface of the SWNT (marked in yellow) are within the range of vdW interaction. Under the action of vdW force, the SWNT moves toward the AgNW in the radial direction to decrease the gap between the SWNT and AgNW surface as much as possible (Figure 5b), which is the premise for the subsequent interfacial behavior. Stage II: Simultaneously, as shown in Figure 5c, the Ag atoms in front of the right side of the SWNT (marked in black-green), in the lower region of the AgNW with a high

Figure 4. Potential and nonbond energies, with the corresponding atomic configurations in different stages as functions of the simulation time during interaction between unsaturated (15, 15) SWNT and AgNW at the temperature of 500 K.

shows the variation trend of the potential energy and nonbond energy with the simulation time. The nonbond energy transitions to a stage of rapid descent, which causes a corresponding change in the potential energy. Simultaneously, the rapid decrease in the nonbond energy appears in the time interval between 320 and 350 ps, which corresponds to the

Figure 5. The interaction model for interfacial contact behavior of the side-to-side biaxial SWNT−AgNW heterostructure based on the vdW interaction: (a) the initial contact atomic configuration; (b) the movement behavior of the SWNT toward the AgNW in the radial direction; (c) the movement behavior of the SWNT along the AgNW periphery in the axial direction; (d) the deformation behavior of the SWNT; and (e) the final interfacial contact configuration with the SWNT completely collapsing and adhering firmly to the AgNW surface. E


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Figure 6. (a) Atomic model/initial configuration and a series of snapshots for instantaneous side-to-side biaxial heterogeneous contact configurations at (b) 300 ps, (c) 600 ps, (d) 610 ps, (e) 620 ps, (f) 630 ps, (g) 640 ps, (h) 650 ps, (i) 660 ps, (j) 700 ps, (k) 800 ps, and (l) 1 ns between the AgNW and the (15, 15) SWNT with hydrogen saturation at both ends of the nanotube at the temperature of 500 K.

Figure 7. Final atomic configurations of the side-to-side biaxial heterogeneous contact structures between unsaturated (15, 15) SWNT and AgNW at different temperatures of (a) 300 K and (b) 800 K and the energy analysis including (c) the binding energy versus temperature and (d) the energy differences (ΔEpotential, ΔEvalence, ΔEcross‑term, ΔEnonbond) between the final atomic configuration and the initial atomic configuration versus different temperatures.

surface energy, can attract the following unsaturated dangling C atoms in the upper-right portion of the right edge of the SWNT (marked in orange). This situation is also applicable to the Ag atoms in the lower-left portion of the AgNW end (marked in black green) and the closely following C atoms on the upper surface of the SWNT (marked in orange). Under the vdW interaction, the SWNT moves along the AgNW periphery in the axial direction. Stage III. The C atoms on the upper surface of the SWNT are also attracted by the Ag atoms on the lower part of the AgNW periphery, as shown in Figure 5d. The interactions between the Ag atoms and C atoms within the range of atomic interaction gradually increase with increasing

distance from the central position, mediated mainly by the vdW force. Then, when the vdW force is large enough, the deformation occurs and the nanotube approaches the nanowire periphery and the cross-sectional shape becomes an inverted saddle structure. During the continuous deformation process of the nanotube, the number of C atoms that interact with the Ag atoms can increase, which can accelerate the complete collapse process with a domino effect. The collapse deformation is a result of the π−π stacking effect between the upper and lower layers of the substantially deformed nanotube. Next, a complete collapse occurs, and a contact configuration is subsequently produced, as shown in Figure 5e, F


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Figure 8. Final atomic configurations of the side-to-side biaxial heterogeneous contact structures between the AgNW and different structure types of the unsaturated SWNTs of (a)−(d) armchair, (e)−(h) zigzag, and (i)−(l) chiral, with the corresponding atomic structures of intrinsic circular form (the illustrations and their enlarged views are represented by display style of ball and stick) at the temperature of 500 K.

with the vdW interaction playing an important role in the interfacial contact behavior of the SWNT−AgNW composite structure. To further verify the effect of the dangling C atoms at both ends of the nanotube on the interfacial contact behavior, a series of atomic contact configurations between AgNW and (15, 15) SWNT with hydrogen saturation at both ends of the nanotube (Figure 6) were examined. The nanotube first moves along the nanowire periphery, then catches up with nanowire, and finally collapses into the bilayer graphene-like structure before tightly adhering to the nanowire periphery, which is similar to the contact behavior in the unsaturated SWNT−

AgNW system shown in Figure 1. However, the nanotube moves more slowly along the nanowire surface and the time of collapse is delayed until 610 ps, which continues for 50 ps. Based on the interaction model and analysis of the interfacial contact behavior shown in Figure 5, the unsaturated dangling C atoms affect the contact behavior to a certain extent. However, this effect is not the primary mechanism of the nanotube collapse because it involves only a very small proportion of the dangling C atoms of the nanotube. Thus, the vdW force between all interacting C and Ag atoms plays an important role in the movement and collapse behavior of the G


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The Journal of Physical Chemistry C Table 1. A Series of SWNT Used in the Simulations chiral index (n, m)

diameter (nm)

length (nm)

number of C atoms of SWNT

number of dangling C atoms at each end of SWNT

chiral angle θ (deg)

structure type

band structure

(5, 5) (10, 10) (15, 15) (30, 30) (50, 50) (24, 0) (26, 0) (27, 0) (29, 0) (18, 12) (20, 10) (27, 9) (28, 8)

0.678 1.356 2.034 4.068 6.78 1.879 2.036 2.114 2.27 2.048 2.071 2.54 2.563

8.116 8.116 8.116 8.116 8.116 8.094 8.094 8.094 8.094 9.284 7.89 7.68 6.974

660 1320 1980 3960 6600 1824 1976 2052 2204 2280 1960 2340 2144

10 20 30 60 100 24 26 27 29 30 30 36 36

30 30 30 30 30 0 0 0 0 23.41 19.11 13.9 22.85

armchair armchair armchair armchair armchair zigzag zigzag zigzag zigzag chirality chirality chirality chirality

metallic metallic metallic metallic metallic metallic semiconducting metallic semiconducting metallic semiconducting metallic semiconducting

be classified into three different types, armchair, zigzag, and chiral nanotubes. The armchair carbon nanotubes are all metallic, and the zigzag nanotubes and chiral nanotubes have metallic and semiconducting properties. It is difficult to discern which factors are responsible for their behavior. Figure 8 shows the final atomic configurations between the AgNW and the different unsaturated SWNTs, which are classified by diameter, from small to large, according to the types of structure, i.e., armchair, zigzag, and chiral; the structural details of the SWNT are listed in Table 1. As shown in Figure 8, all types of SWNTs can move along the periphery of the nanowire and keep pace with the AgNW and the collapse behavior does not appear in all SWNT−AgNW composite systems. For instance, for the armchair SWNT−AgNW systems that have the same length as the SWNT, the nanotube diameter size is controlled by varying the chiral index (m, n) to change the diameter ratio of the SWNT to the AgNW based on fixing the AgNW diameter at 2 nm. Only (5, 5) metallic armchair nanotube does not collapse and still maintains excellent intrinsic circular form, as shown in Figure 8a. There is no rapid decline in the nonbond energy (vdW energy), which differs from the collapse situation. When the nanotube diameter is very small, the mechanical strength of the hollow cylindrical structure is relatively high and the vdW interaction is too small to induce large structural changes or collapse. Similarly, the nanotubes, with smaller mechanical strengths due to their larger diameter sizes, collapse more easily under a relatively larger vdW interaction. Similar interfacial contact behavior occurs in the zigzag and chiral SWNT−AgNW systems. However, in the case of similar diameter size, the collapse behavior does not occur in all SWNT−AgNW systems with different structure types. For example, the diameter of the SWNT is similar to that of the nanowire in the (15, 15) armchair, (26, 0)/(27, 0) zigzag, and (18, 12)/(20, 10) chiral SWNT−AgNW systems. The collapse behavior does not occur in all SWNT−AgNW composite systems but only in the (15,15)/(27, 0) SWNT−AgNW systems with large energy differences, as shown in the Supporting Information. This behavior is related to the spatial arrangement and the positional relation of the C atoms in different SWNTs with the armchair, zigzag, and chiral structures, which can affect the different mechanical strengths and deformations of the nanotubes. As shown in Figure 8, the most complex atomic network structures that have high structural stability and strength exist in the chiral SWNT− AgNW systems, while, successively, in zigzag SWNT−AgNW

SWNT−AgNW system, as based on the analysis seen in Figure 5. 3.4. Influence of Temperature on the Interfacial Contact Behavior. To further explore how some factors affect the interfacial contact behavior and the atomic configurations between the SWNT and AgNW, the final atomic configurations at the other temperatures of 300 and 800 K (Figure 7a,b) were examined. SWNT can move along the periphery of the nanowire and keep pace with AgNW at different temperatures. Similarly, SWNT collapses and adheres firmly to the AgNW periphery at a relatively low temperature of 300 K. In sharp contrast to the behavior shown in Figure 7a, the nanotube collapse behavior does not occur at a relatively high temperature of 800 K. As shown in Figure 7c, the binding energies of the SWNT−AgNW system are 2141.14, 1883.23, and 889.97 kcal/mol at 300, 500, and 800 K, respectively. This variation trend indicates that the binding energy is inversely proportional to temperature, which is related to the thermodynamic properties of the atoms. Based on the atomic morphology of the AgNW, when the ambient temperature reaches 800 K, the nanowire surface melts into a liquid state with loose and irregular atomic structures, exhibiting an obvious thermodynamic behavior. This increased temperature can weaken the metallic bonding effect and thus influence the interaction between the Ag atoms and the C atoms. Conversely, the nanowire maintains a good morphology with compact and regular atomic structures at 300 and 500 K, and it exhibits a strong metallic bonding that affects the SWNT atomic configurations. Furthermore, Figure 7d illustrates the different energy differences (ΔEpotential, ΔEvalence, ΔEcross‑term, ΔEnonbond) between the final atomic configuration and the initial atomic configuration versus temperature. The nonbond interaction energy changes from a negative value at relatively low temperatures of 300 and 500 K to a positive value at a relatively high temperature of 800 K, indicating that the vdW interactions are substantially altered under different temperatures and thus induce different interfacial contact behaviors and atomic configurations. 3.5. Influence of the Structural Characteristics of the Nanotube on the Interfacial Contact Behavior. Following the above-mentioned interfacial contact atomic configuration results, the different SWNT−AgNW structure types were considered in the simulations to investigate whether a similar configuration phenomenon is exhibited in other types of SWNT−AgNW composite systems. The SWNT structure can H


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is different from that in the unsaturated (27, 0) nanotube. Therefore, the unsaturated dangling C atoms have a large impact on the behavior of the nanotube when their structural characteristics are similar to those of the nanotube. As shown by the shapes of the collapsed nanotubes in Figure 8, if the SWNT diameter is relatively large, such as (30, 30) and (50, 50), the SWNTs not only collapse on the nanowire periphery but also scroll and wrap around the nanowire to form core/shell hybrid structures. Figure 10 shows a series of atomic contact configurations at 10 ps intervals during the interaction process. The nanotube begins to significantly deform at 490 ps (Figure 10a), and then the nanotube collapses into the bilayer graphene-like structure while scrolling on the nanowire periphery. Eventually, the formed bilayer graphene-like structure wraps around the AgNW to form core/ shell hybrid structures (Figure 10i). From another point of view, the formed bilayer graphene-like structure has been transformed into another form of double-walled carbon nanotubes (DWNTs), which is approximately regarded as the process of graphene curling into a carbon nanotube. Therefore, the process includes the transformations from SWNT to bilayer graphene-like structure to DWNT. The interfacial contact behavior and core/shell hybrid atomic configurations can provide valuable theoretical guidance for designing and fabricating functional composite materials with broad potential applications.

systems and armchair SWNT−AgNW systems. As the most stable chiral SWNT−AgNW systems (Figure 8), only the SWNTs with larger diameters can collapse under a relatively larger vdW interaction. Additionally, because these SWNTs have a similar number of dangling C atoms at each end in the simulations, the interaction between the dangling C atoms and the AgNW is not the major factor in the collapse of the nanotube. Thus, the structure type and nanotube diameter affect the collapse of the SWNTs. In Figure 8, it can also be seen that the vdW force is in a range that ensures the collapse of the unsaturated (27, 0) nanotube and the noncollapse of the unsaturated (26, 0) nanotube. As also shown in Figure 8, the unsaturated (26, 0) and (27, 0) nanotubes, which have the same type of structure, have similar diameters of 2.036 and 2.114 nm, respectively. However, even if the structure type and diameter are similar, the unsaturated dangling C atoms can have different effects on the behavior of nanotubes. The corresponding simulations on the H-saturated SWNT−AgNW systems were performed, and the initial and final atomic configurations are shown in Figure 9. Based on the magnitude

4. CONCLUSIONS In summary, the molecular dynamic simulations provided a direct observational method for exploring the interfacial contact behavior of the side-to-side biaxial heterostructure between single-walled carbon nanotubes and silver nanowires on an atomic scale. The results of the simulations indicate that the vdW interaction plays an important role in the interfacial contact process, which causes the nanotube to move along the nanowire periphery and keeps pace with the silver nanowires. If the vdW energy of the SWNT−AgNW system dramatically decreases in a very short time, it can spontaneously induce the nanotube to collapse. Moreover, whether carbon nanotubes exhibit collapse behavior depends on some complex factors. For the armchair nanotubes, the collapse and domino effect readily occur to form the bilayer graphene-like structures with

Figure 9. Initial and final atomic configurations for (a) saturated (26, 0) SWNT−AgNW and (b) saturated (27, 0) SWNT−AgNW systems at the temperature of 500 K, respectively.

of the vdW force, the (26, 0) SWNT cannot collapse due to the lack of unsaturated dangling atoms. The noncollapse phenomenon exists in the H-saturated (27, 0) nanotube, which

Figure 10. Snapshots of the collapse behavior of the SWNT with atomic contact configurations at times (a) 490 ps, (b) 500 ps, (c) 510 ps, (d) 520 ps, (e) 530 ps, (f) 540 ps, (g) 550 ps, and (i) 560 ps between the unsaturated (50, 50) SWNT and the AgNW at the temperature of 500 K. I


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The Journal of Physical Chemistry C a face-to-face π−π stacking effect, adhering firmly to the nanowire surface at relatively low temperatures, except for nanotubes with very small diameters or SWNT−AgNW systems in high-temperature environments. The collapse behavior is also related to the spatial arrangement and positional relation of the C atoms in the SWNTs with the armchair, zigzag, and chiral structures, which can affect the mechanical strength and deformations of the nanotube. The most complex atomic network structures that possess better structural stability and strength exist in the chiral SWNT− AgNW systems, while the zigzag SWNT−AgNW systems and the armchair SWNT−AgNW systems follow successively. In cases of similar nanotube structure characteristics, the unsaturated dangling C atoms have a large impact on the behavior of the nanotube. When the diameter of the armchair nanotube is very large, the formed bilayer graphene-like structure scrolls onto the nanowire periphery and wraps around the nanowire to form a core/shell hybrid structure, which eventually transform into a double-walled carbon nanotube structure. Because of their interfacial contact behavior and final atomic configurations, these kinds of systems may have broad potential applications. For instance, the behavior of the nanotube moving along the nanowire surface provides an insight into the fabrication of nanomotors based on vdW as a driving force. The behavior of the nanotube collapsing, scrolling, and wrapping around a nanowire is a promising method for designing and fabricating core/shell heterostructures as a new type of building block that can be excepted to be an important part of nanoelectronic devices, functional composite materials, conductive films, heterogeneous catalysis, and energy storage.

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(2016QNRC001), Fundamental Research Funds for the Central Universities (xtr042019001), China Postdoctoral Science Foundation (2019T120898, 2018M640976), Open Foundation of the State Key Laboratory of Fluid Power and Mechatronic Systems (GZKF-2018016), and Open Research Fund of State Key Laboratory of Transient Optics and Photonics (SKLST201704). All the authors gratefully acknowledge their support.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.9b05181. Molecular dynamics simulation parameters and interaction energy data; atomic configurations of the SWNT−AgNW heterostructure (PDF) Video 1 (MP4) Video 2 (MP4) Video 3 (MP4)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J.C.). *E-mail: [email protected] (H.T.). *E-mail: [email protected] (X.M.). ORCID

Jianlei Cui: 0000-0002-5760-509X Wenjun Wang: 0000-0002-2562-4077 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by National Key Research and Development Program of China (2017YFB1104900), National Natural Science Foundation of China (51875450, 51505371), Shaanxi Provincial Key Research and Development Program (2019ZDLGY01-09), JSPS KAKENHI (18F18356), Young Elite Scientists Sponsorship program by CAST J


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Atomic Scale Simulation of the Contact Behaviour and Mechanism of

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