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Single-Walled Carbon Nanotube Engendered Pseudo-1D Morphologies of Silver Nanowire Sunil Kumar,*,† Vikas Chandra Srivastava,† Gopi Kishor Mandal,† Sudip Kumar Pattanayek,‡ and Kanai Lal Sahoo† †

CSIR-National Metallurgical Laboratory, Jamshedpur, India 831007 Indian Institute of Technology, New Delhi, India 110016

‡

ABSTRACT: Silver nanowires show enhancement in desired properties, due to high surface area and high aspect ratio, which increases the possibility in design and development of many advanced optoelectronic devices. The organization of silver atoms and the morphology of cylindrical nanowire highly influence the desired properties at various physical conditions during applications. Therefore, the synthesis of nanowires with desired atomic organization becomes essential. In the present study, a pseudo-1D morphology of silver nanowires, encapsulated into a Single Wall Carbon Nanotubes (SWCNTs), has been investigated by employing Molecular Dynamics (MD) simulation. At high temperature (T = 1500 K), molten silver encapsulated into a SWCNT attaining a low energy state followed by restricted thermal motion or vibration. With an increase in SWCNT diameter, various unique pseudo-1D morphologies of encapsulated silver atoms were observed during cooling from 1500 to 10 K. Silver atoms, encapsulated into a SWCNT having a diameter of 10.85 Å, mainly reveal 5-fold symmetrical icosahedral (ico) having pseudo-1D morphology. With a further increase in diameter (d = 40.68 Å) of SWCNT, a decagonal pattern consisting of face centered cubic (fcc), hexagonal closed packed (hcp) and body centered cubic (bcc) was evolved due to nonhomogenous packing of silver atoms into SWCNT. Simulation results indicate that the diameter of SWCNT is the one of the major factors controlling the pseudo-1D morphology of silver nanowire. The present theoretical investigations provide a guideline and enhance the current understanding related to solid state physical phenomena of metal nanowires synthesis. biosensing,21−23 antibacterial applications,24,25 and as additives in composite fibers.26 Various experimental27−44 and simulation45−52 methodologies have been developed for the synthesis and characterization of metal nanowire. Ajayan27 developed an experimental ex-situ technique for the inclusion of metals into multiwalled carbon nanotubes (MWCNT), for the synthesis of metal nanowires, with an encapsulation yield of up to 90%. Later, Ajayan and Ebbesen28 demonstrated that this approach can be successfully applied to fill SWCNTs without performing the preliminary opening of its ends. Over the years, various research groups came up with different chemical approaches for the synthesis of 1D-nanostructureed silver nanowire. For example, silver nanowires can be synthesized by reducing silver nitrate (AgNO3) in the presence of silver bromide (AgBr) nanocrystallites29 or by arc discharge between two silver electrodes immersed in an aqueous sodium nitrate (NaNO3) solution.30 Ultraviolet irradiation photoreduction technique has been used to synthesize silver nanowires by passing ultraviolet light into an aqueous silver nitrate (AgNO3)

1. INTRODUCTION Encapsulation of a variety of molecules, such as water,1,2 polymers,3−6 biopolymers,7−9 and metals10−15 within SingleWalled Carbon Nanotubes (SWCNT) has been attracting much interest recently, not only due to their superior composite properties, but also the pseudo-1D organization of encapsulated materials. In addition, encapsulation of fullerenes (C60),16 deoxyribonucleic acid (DNA),17,18 and graphene nanoribbons19,20 has also generated a significance in the development of novel functionalized nanomaterials for the application in biomedical and electronic devices/sensors, high performance nanocomposites, catalyst for electrochemical reactions, solar cells, and so on. Furthermore, the high temperature stability of metallic compounds can also be significantly improved when encapsulated within SWCNT, as the hexagonal arrangement of carbon casing inhibits oxidation.17 Similarly, silver atoms can also be encapsulated into the cylindrical cavity of SWCNT, which opens the possibility of synthesizing ultrathin silver nanowires. Silver nanowires are the class of materials that have drawn considerable attention due to their fascinating electrical, thermal, and optical properties, which can be constructively utilized in optical, electrochemical, © 2017 American Chemical Society

Received: June 18, 2017 Revised: August 10, 2017 Published: August 29, 2017 20468

DOI: 10.1021/acs.jpcc.7b05973 J. Phys. Chem. C 2017, 121, 20468−20480

Article

The Journal of Physical Chemistry C

pseudo-1D morphology of silver nanowire inside the SWCNT have not been attempted earlier in the light of published literature.

solution in the presence of poly(vinyl alcohol), which is known as polyol process.31−33 Sun et al.32 have used polyol process for the large-scale synthesis of silver nanowires, with uniform diameters in the range of 30−40 nm and lengths up to 50 μm. The properties of silver nanowires produced by all these methods generally showed irregular morphologies, polycrystallinity, low aspect ratio, and poor mechanical strength. In contrast to these methods, the template (nanochannels) directed synthesis40 engenders silver nanowires, having pseudo-1D morphology of atoms, with better and consistent mechanical, thermal, and electrical peroperties. Various types of template or nanochannels, such as macroporous membranes,34 mesoporous materials,35 carbon nanotubes,27,28,36 DNA chains,37 rod-shaped micelles,38 and steps or edges on solid substrates,39 have been successfully used for the synthesis of metal nanowires. In the present investigation, therefore, we have used SWCNT as a template to study the encapsulation behavior and evolution of pseudo-1D morphology of silver atoms during cooling from high temperature (1500 K) to extremely low temperature (10 K) by using molecular dynamics (MD) simulation methodology. The MD simulation technique has been used by a few research groups to investigate various properties of metal nanowire.45−50 Most of such reported studies on metal nanowires are based on the estimation of mechanical, electrical, and thermal properties.45−47 A few studies are also available on the melting and solidification of metal nanowire with and without templates (nanochannels). Chen et al.48 have studied the melting behavior of silver nanowire, with 5-fold twinned boundaries created without using any template, of various diameter ranging from 1.6 to 10 nm. They observed a decrease in melting temperature of silver nanowire with decreasing diameter, by employing MD simulation. The melting temperature of silver nanowire has been found to be 1250 K (diameter = 10 nm) and 790 K (diameter = 1.6 nm). On the contrary, Arcidiacono et al.49 have studied the solidification of gold atoms inside the SWCNT template. They have shown that the solidification temperature of gold nanoparticles depends on their size and diameter of nanotubes. Choi et al.50 have studied the encapsulation of copper into SWCNT by cooling the copper melt (cooling rate 1 K/ps) from 700 to 10 K. They have reported that copper atoms organize themselves in a face-centerd cubic ( fcc) structure inside the SWCNT, as occurs in bulk material. Despite these studies on encapsulation of Ag and other materials, the issues related to the structure and the stability of pseudo-1D morphology of silver nanowires with a diameter of the cylindrical template (SWCNT) remained unanswered. The present study is an attempt to have an insight on the structure of silver nanowire inside a solid cylindrical template (SWCNT) and the structural transition with the various template diameter. In this investigation, MD simulation has been used for the study of pseudo-1D morphology of silver nanowire considering SWCNT as hard cylindrical template. The simulation system consists of multiple steps, such as, melting of silver at 1500 K, encapsulation of silver atoms into SWCNT (at 1500 K), and cooling of silver from 1500 to 10 K (cooling rate 0.1 K/ps). The radial distribution function, adaptive common neighbor analysis, mean square displacement, and energy contributions have been used to characterize the encapsulation and pseudo1D morphology of silver nanowire inside the SWCNT. To the best of the knowledge of the authors, such extensive investigations addressing a multitude of coupled phenomenological issues such as encapsulation process and evolution of

2. SIMULATION DETAILS Constant temperature molecular dynamics simulation51 has been implemented to study the encapsulation of silver atoms into SWCNT of various diameters and their atomic organization therein, as given in Table 1. A generalized Finnis Table 1. Parameters for Embedded Atoms Method (EAM) Potential Due to Finnis−Sinclair (FS)5253 for Silver Atoms m

n

ε (eV)

c

a (Å)

6

12

0.025415

144.41

4.09

and Sinclair52,53 form of embedded atom method (EAM) potentials has been employed between the silver atoms. The total energy of silver atoms, EAg−Ag is given by ⎡

EAg−Ag =

1 ⎣2

∑ ε⎢⎢ ∑ V (rij) − c i

j≠i

⎤ ρi ⎥ ⎥⎦

(1)

⎛ a ⎞n V (rij) = ⎜⎜ ⎟⎟ ⎝ rij ⎠

(2)

⎛ a ⎞m ⎜ ρi = ∑ ⎜ ⎟⎟ r j ≠ i ⎝ ij ⎠

(3)

where rij is the distance between ith and jth silver atom, ε and a energy and lattice parameters, and n, m, and c are the positive constants for silver atoms. V(rij) is the pair potential to account for the repulsion between ith and jth silver atom, and ρi the local density associated with silver atom i. Equations 1−3 can be generalized to derive the Hamiltonian pertaining to silver atoms in the following Finnis-Sinclair form ⎡ ⎤ ⎡ ⎤1/2 1⎢ ⎥ ⎢ H= ∑ ∑ pi ̂ pĵ V (rij)⎥ − d ∑ pi ̂ ⎢∑ pĵ ⌀(rij)⎥⎥ 2 ⎣⎢ i ≠ j ⎦ ⎣ j≠i ⎦ i

(4)

The site occupancy operators p̂i for silver atoms are defined as follows ⎧1, if site i is occupied by a silver atom pi ̂ = ⎨ ⎩ 0, if site i is not occupied by a silver atom

(5)

⎡ a ⎤n V (r ) = ε ⎢ ⎥ ⎣r ⎦

(6)

⎡a⎤ ⌀ (r ) = ε ⎢ ⎥ ⎣r ⎦

(7)

⎪

⎪

m

The constant d is defined as follows d=ε×c

(8)

The values of ε, a, c, m, and n for the silver atoms are given in Table 1. The interatomic potential energy between silver and carbon atoms of SWCNT have been implemented through 12−6 Lennard-Jones potential, as given in the following equation. 20469


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The Journal of Physical Chemistry C ⎡⎛ σAg−C ⎞12 ⎛ σAg−C ⎞6 ⎤ ⎟ ⎟ ⎥ E LJ = 4εAg−C⎢⎜ −⎜ ⎝ r ⎠⎦ ⎣⎝ r ⎠

number of silver atoms (NAg) and size of armchair (n, n) SWCNT are listed in Table 2. Molecular dynamics simulation (9)

Table 2. Details of the Simulation System Such as Surface Architecture, Diameter, and Length of SWCNT, along with the Number of Silver Atoms (NAg) for the Synthesis of a Silver Nanowire

where εAg−C and σAg−C are the Lennard-Jones parameters for energy minimum or well-depth and equilibrium interatomic distance at null potential, respectively. The subscripts denote the silver (Ag) atom and carbon (C) atoms of graphene. The values of εAg−C = 0.0301 eV and σAg−C = 3.006 have been used for silver and SWCNT.54 The Newtonian equation of motion of a silver atom, i, of mass (w), due to the external forces arising from the sum of the various potentials, E can be written as

w×

d 2ri dt 2

= Fi = −∇i E

(10)

where Fi is force on the ith silver atom. The equation of motion (eq 10) is integrated according to the velocity Verlet algorithm55 with a time step of 2 fs, as shown in eqs 11 and 12. ri(t + Δt ) = ri(t ) + vi(t )Δt + vi(t + Δt ) = vi(t ) +

1 (Δt )2 ai(t + Δt ) 2

1 Δt[ai(t ) + ai(t + Δt )] 2

(11)

armchair (n, m) SWCNT

diameter of SWCNT (Å)

length of SWCNT (Å)

No. of carbon atoms in a SWCNT

No. of silver atoms (NAg)

(8,8)SWCNT (9,9)SWCNT (10,10)SWCNT (12,12)SWCNT (15,15)SWCNT (20,20)SWCNT (25,25)SWCNT (30,30)SWCNT (35,35)SWCNT (40,40)SWCNT

10.85 12.20 13.56 16.24 20.30 27.12 33.85 40.68 47.43 54.00

100 100 100 100 100 100 100 100 100 100

1312 1476 1640 1968 2460 3280 4100 4920 5740 6560

220 320 400 732 1298 2500 4400 6912 10000 13500

for the synthesis and characterization of silver nanowire employs three process steps. In the first step, silver is melted to a fixed temperature (T = 1500 K) and held for 100 ps, which allows the melt to achieve an equilibrium state. The experimentally determined melting temperature of bulk silver is 1234 K.48 A comparatively high temperature of 1500 K was selected for melting so as to achieve a truly randomized and homogeneous liquid. The carbonaceous nanomaterial, such as Single Walled Carbon Nanotube (SWCNT) and graphene, is able to withstand a temperature of up to 3300 K (estimated by molecular dynamics simulation)62 and 2073 K (estimated experimentally).63 In the second step, silver atoms are allowed to migrate into the SWCNT at a temperature of 1500 K for 1000 ps, which is named as encapsulation processes. In the third and final step, the temperature of the siver decreases from 1500 to 10 K at a cooling rate of 0.1 K/ps, in which silver shows phase transition from liquid to solid. In contrast to the reported literature,64−66 a comparatively slower cooling rate for solidification has been adopted in the present study so as to efficiently capture the evolution of the nanocrystalline structure of encapsulated silver nanowire. Most of the studies64−66 have used a cooling rate ranging from 0.1 to 10 K/ps for the crystallization process of pure metals. 2.2. Radial Distribution Function g(χ). The probability of finding a silver atom at a distance χ from an average central silver atom is calculated through the radial distribution function g(χ)67 defined by the following eq 13:

(12)

Here, vi and ai are the velocity and acceleration of the ith silver atom. The MD simulation is carried out in a NVT ensemble. The Nose-Hoover thermostat56,57 has been implemented to maintain the appropriate temperature of the system along with velocity-Verlet algorithm. Large Scale Atomic/Molecular Massively Parallel Simulator (LAMMPS),58 Open Visualization Tool (OVITO),59,60 and Visual Molecular Dynamics (VMD)61 software packages have been used for molecular dynamics simulation, visualization, and analysis of output data. 2.1. Simulation System and Processing Steps. The simulation system consists of a SWCNT and specified number of silver atoms, as shown in Figure 1. A SWCNT is placed along the Z-direction in the center of the simulation box of dimension (±200 Å, ±200 Å, ±200 Å). The silver atoms with an ideal fcc structure is placed at an end of the SWCNT. The

g (χ ) =

V 2 NAg

NAg

∑ i=1

n(χ ) 4πχ 2 Δχ

(13)

where NAg and V are the total number of silver atoms and volume of the system, respectively. n(χ) is the number of silver atoms in a spherical shell of radius χ and thickness Δχ around the central silver atom. 2.3. Adaptive Common Neighbor Analysis. The adaptive Common Neighbor Analysis (a-CNA)59,60 is used to characterize the organization of silver atoms inside the SWCNT. The a-CNA has the capability to identify the silver atoms belonging to various structure types such as face centered cubic (fcc), hexagonal closed packed (hcp), body

Figure 1. Simulation system consisting of a globule of silver atoms and a SWCNT, which has been placed in the center of the simulation box of size (±200 Å, ±200 Å, ±200 Å). The axis of SWCNT is along the Z-direction. The silver globule, with a temperature of 1500 K, is placed at an open end of SWCNT. 20470


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Figure 2. Snapshots of side and top view of silver during encapsulation process into a SWCNT of diameter d = 40.68 Å, with increasing simulation time at a fixed temperature of 1500 K. Red color arrows are indicating the progress of encapsulation process.

performed by using compute msd command within LAMMPS during each simulation run.

centered cubic (bcc), icosahedra (ico), and others using only the position (x, y, and z coordinates) of each silver atom. In the aCNA analysis, the local crystal structure of silver atoms was recognized by estimation of three integers (ncn, nb, and nlcb) for each central silver atom.64,68−70 ncn is the number of silver atoms that are neighbors to both atoms in the pair, which are identified as common neighbors. nb is the total number of bonds between ncn common neighbors of silver atoms. nlcb is the number of bonds in the longest continuous chain formed by the nb bonds between common neighbors of silver atoms. For the perfect fcc structure, all pairs are of the type (ncn, nb, nlcb) = (4, 2, 1) or simply known as 421. Similarly, for the perfect hcp, 50% integers are of type 422 and other 50% integers are of type 421. Furthermore, bcc structure is consisting of both 441 and 661 types. This algorithm for identification of crystal structure is inbuilt in the Open Visualization Tool (OVITO) software by its developers, which we have used for analysis in this study. OVITO software package, randomly organized silver atoms identified as liquid/ glassy state. However, in the crystalline state of silver, atoms are organized in fcc, bcc, and hcp structure. Icosahedra (ico) organization of atoms are found in a metastable glassy state. 2.4. Mean Square Displacement (MSD). The mean square displacement (MSD)51 of silver atoms are calculated to quantify the atomic mobility in both encapsulation and cooling process. The MSD of silver atoms can be computed by the following eq 14: MSD = |r(t ) − r(0)|2

3. RESULTS AND DISCUSSION The results obtained from the present attempt on MD simulation of silver atom encapsulation within a SWCNT have been presented and discussed. This contains various aspect of the encapsulation, for example, migration into a SWCNT, evolution of pseudo 1-D morphologies of silver

(14)

Figure 3. Variation in potential energies of silver atoms (PEAg−Ag) and between silver and carbon atoms of SWCNT (PEAg−C) during encapsulation process (SWCNT diameter d = 40.68 Å; temperature T = 1500 K).

where r(0) and r(t) are the position of the silver atom at time t = 0 and t = t, respectively. The calculation of MSD has been 20471


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Figure 6. Variation in potential energies of silver atoms (PEAg−Ag) and between silver atoms and carbon atom of SWCNT (PEAg−C) of diameter d = 40.68 Å during cooling from 1500 to 10 K.

Figure 4. Radial distribution function g(χ) of silver atoms during encapsulation process within the SWCNT of diameter d = 40.68 Å at 1500 K.

encapsulation process. The similar observations of encapsulation process has been reported in our previous publications.3,4 During encapsulation process, the potential energies PEAg−C and PEAg−Ag decreases due to gradual increase in contact with the carbon atoms of SWCNT and also with neighboring silver atoms, as shown in Figure 3. The first peak of g(χ) versus χ plot increases slightly during encapsulation process, which depicts the increase in the number of first nearest neighbor atoms (i.e., increase in contact number), as shown in Figure 4. The increase in number of first nearest neighbor will increase the PEAg−Ag. From the energy calculations, it can be concluded that SWCNT provides a favorable potential energy well and a strong hexagonal network of carbon atoms. At high temperature, that is, once the silver is in liquid state, the possibility of decrease in potential energy leads to encapsulation of silver atoms into SWCNT. At the same time, strong hexagonal network of carbon atoms will reduce the thermal vibration of

atoms and the effect of SWCNT diameter on structural transitions. 3.1. Encapsulation of Silver Atoms into SWCNT. Silver atoms (NAg = 6912) are placed near an open end of a SWCNT, diameter d = 40.68 Å, at a temperature of T = 1500 K to study their encapsulation process into cylindrical cavity of SWCNT. Figure 2 depicts the snapshots of temporal evolution of encapsulation process. In the early stage of simulation (up to 250 ps), most of the silver atoms travel along Z-direction (axis of SWCNT) and occupy all the available space inside the SWCNT. A stable encapsulated state of silver atoms inside the SWCNT has been observed after 1000 ps. For a monatomic metal system, the nonbonded interaction potential energy between carbon atoms of SWCNT and silver atoms (PEAg−C), along with intra-atomic potential energy of silver atoms (PEAg−Ag), are expected to be the driving force for the

Figure 5. Snapshots of phase transition from liquid to crystalline state of silver encapsulated into SWCNT of diameter d = 40.68 Å during cooling from 1500 to 10 K. Blue, red, green, and yellow color atoms pertain to bcc, hcp, fcc, and ico structure, respectively. For clear visualization, atoms with random organization have not been shown in the snapshot. SWCNT has not been shown in right most snapshot due to clear visualization of solidified silver. Red color arrows are indicating the progress of phase transition. 20472


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Figure 7. Liquid to solid phase transition of pure silver during cooling from 1500 to 10 K: (a) snapshot of structure evolution and (b) variation of fcc, hcp, bcc, ico, and other atoms during cooling from 1500 to 10 K. Blue, red, green, and yellow color atoms pertain to bcc, hcp, fcc, and ico structures, respectively. Red color arrows are indicating the progress of phase transition.

during cooling process, as shown in Figure 6. With decreasing temperature, reduction in thermal vibration facilitates the crystallization process inside the SWCNT. A sharp decreases in both the potential enegies is indicative of the phase transition from liquid to crystalline phase. This type of energy change is also observed during crystallization of pure metals in bulk.71−73 We have done simulation of pure silver (as in bulk) to estimate the crystallization temperature of silver. It has been found that silver start to crystallize in fcc structure with twin plane of hcp at nearly 850 ± 15 K (diameter of silver globule d ≈ 6 nm, number of silver atom = 6912), as shown in Figure 7a,b. Tian et al.74 have also found similar crystallization temperature, T = 862 K (for 5000 silver atoms) and T = 871 K (for 10000 silver atoms) of silver in bulk by using molecular dynamics simulation. However, the crystallization temperature of encapsulated silver into SWCNT depicts 1290 K. It can be concluded that encapsulation of silver into SWCNT increases the crystallization temperature significantly.

encapsulated silver atoms. These observations clearly reveal that silver atoms encapsulate into the SWCNT to reach a stable and lower energy state, compared to the their energy state in bulk, that is, outside the SWCNT. 3.2. Organization of Silver Atoms inside the SWCNT. During the cooling process, the temperature of silver decreases from 1500 to 10 K and silver atoms form a one-dimensional clylindrical structure within the SWCNT at low temperature, as shown in Figure 5. At a temperature of 1275 ± 25 K, which is slightly higher than the melting temperature of 1234 K,48,81,82 silver atoms witness phase transition and commences arranging themselves into crystalline phase, due to the presence of the SWCNT wall.49 Further, as the temperature decreases, silver atoms organize into a pseudo-1D morphology, which consist of various structural organizations such as fcc, bcc, hcp, ico, and others. The average potential energy of silver atoms decreases due to both intra-atomic interactions of silver atoms (PEAg−Ag) and also between silver and carbon atoms of SWCNT (PEAg−C) 20473


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Figure 8. Radial distribution function g(χ) of silver atoms encapsulated within the SWCNT of diameter d = 40.68 Å during cooling from 1500 to 10 K. The first nearest neighbors (1st N.N.) of silver atoms is indicated by arrows. Amorphous or liquid, transition and crystalline regions are shown by black, red, and green color lines at respective temperatures.

Figure 10. Organization of silver atoms inside the SWCNT of diameter d = 40.68 Å: (a) pattern of bcc, fcc, and hcp atoms, (b) decagon symmetry, and (c) 5-fold symmetry of twinned plane of hcp atoms at the end of SWCNT. Red, blue, green, and yellow color atoms indicate hcp, bcc, fcc, and ico atoms.

Figure 9. Evolution of atomic content (in %) corresponding to fcc, hcp, bcc, and ico structures of silver atoms encapsulated into SWCNT of diameter d = 40.68 Å during cooling from 1500 to 10 K.

Figure 9 shows the evolution of ico, fcc, bcc, and hcp atoms during the cooling from 1500 to 10 K. It is observed that, at high temperatures (T > 1290 K), most of silver atoms are in the liquid state, which is organized randomly. Further, with decrease in temperature below (T < 1290 K), a phase transition from liquid to solid (crystalline) is observed. The figure clearly depicts that bcc is the most favorable intermediate organization during phase transition at the crystallization temperature T = 1290 K. Similar bcc configuration has also been reported by Desgranges and Delhommelle77 for the crystallization of pure metal in bulk. The solid crystalline structure mainly consists of fcc, bcc, and hcp atom along with scattered random ico atoms. At low temperatures, that is, T < 300 K, most of the silver atoms organize themselves in a stable combination of fcc, bcc, and hcp compared to pure fcc in bulk. This difference in organization of atoms encapsulated within cylindrical SWCNT may be attributed to the nonhomogenous distribution of energy evolved due to presence of carbon atoms of the SWCNT. Figure 10 a depicts the snapshot of internal view of organization of silver atoms inside the SWCNT and shows their evolution into an interesting cylindrical pattern consisting of long chains of fcc and bcc atoms along the SWCNT axis.

The radial distribution function among silver atoms, g(χ), has been calculated and is shown in Figure 8. At high temperatures i.e. 1500 and 1300 K, a peak of g(χ) is present only at χ = 2.89 Å, which is due to a short-range ordered structure. This peak corresponds to the first nearest neighbor of silver atoms that is 2

found to be at a × 2 = 2.892 Å , where a is the lattice parameter (a = 4.090 Å). However, χ > 2.89 Å, no distinct and identifiable peak was found, which may be due to the loss of long-range ordered correlations. These observations show that the silver atoms exhibit only short-range ordered correlations at high temperature. At moderate temperatures, the peaks of g(χ) are identified to evolve gradually at various values of χ due to phase transition from liquid to crystalline state. At low temperatures, that is, 300 and 10 K, distinct peaks of g(χ) are found at χ = 2.89 and 4.685 Å, which corresponds to first and second nearest neighbors of silver atoms, respectively. These observations indicate that the silver atoms exhibit both shortand long-range ordered correlations at low temperature. The similar description of first and second nearest neighbor of atoms in bulk has been reported in literature.75,76 20474


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Figure 11. (a) Landscape view of potential energy of silver atoms and (b) average potential energy distributions of fcc, hcp, bcc, ico, and other atoms. Each distribution has been normalized by the unit areas under each curve.

diameters were selected for the purpose, as listed in Table 2. The snapshots of silver atoms encapsulated into a SWCNT of different diameters are shown in Figure 12a−e that includes the snapshot from the SWCNT with diameter 40.68 Å also (Figure 12b) that has been described in the previous section. Similar to the atomic organization within 40.68 Å SWCNT, 54.60 Å SWCNT also shows fcc and bcc dominated outer layer, hcp in the interior of the nanowire and 5-fold twinned plane. However, at some locations, outer layers also depicted the presence of hcp atoms, and ico atoms were not observed in the center of the nanowire. This may be attributed to the larger system size and less time for energy equilibration at a cooling rate of 0.1 K/ps. The twinned plane consists of hcp silver atoms. However, the organization inside the 32.50 Å diameter SWCNT shows the decagonal symmetry with mainly hcp atoms, both at the surface and interior, as shown in Figure 12c. In this case also, no ico atom was observed at the central axis of the nanowire. The organization within SWCNT of diameter 20.30 Å cosists of ico atoms at the central axis of SWCNT and hcp atoms at the surface and the interior. In contrast, encapsulation of silver atoms into SWCNT of diameter 20.30 and 10.85 Å showed evolution of ico-atoms at the central axis. In addition to ico-atoms, a few hcp atoms also evolve inside the SWCNT to 20.30 Å. However, in these small diameter SWCNTs, atoms in fcc and bcc configurations have not been seen. Figure 13a,b depicts the organization of silver atoms inside the SWCNT of diameter d = 10.85 Å during cooling from 1500 to 10 K. We have observed the silver atom with icosahedra (ico) gradually evolve as temperature decreases below ≈1100 K. However, atoms with structural organization with fcc, bcc, and hcp does not appear throughout the cooling process. The organization of silver atoms inside the SWCNT of diameter d = 10.85 Å is given in Figure 14a−c. Figure 14a and b show the full view of the encapsulated atoms with and without the SWCNT, respectively. It is obvious from the figures that the atoms organize with 5-fold symmetry along the central axis of SWCNT. This 5-fold symmetry seems to evolve by the stacking of two alternating atomic layers containing a single atom and a five-atom ring, which form a 1−5−1−5−1 stacking structure

Figure 10b shows a slice cut top view of the 1D nanowire depicting a decagonal pattern of silver atom organization inside the SWCNT of diameter d = 40.68 Å. The decagon pattern consists of bcc, fcc, and hcp atoms. The ensuing organizational pattern of atoms inside the SWCNT can be divided into 10 similar triangular shape geometries, where fcc atoms organize at the base of the triangle, hcp at the interior region of the triangle, and bcc and ico atoms at the corners. However, the atomic organization at the ends of SWCNT evolves into a 5-fold twinned symmetry (Figure 10c), which consists of twin planes of hcp atoms due to thermally induced stress concentration at the solid−liquid interface. 5-Fold twinned structure has been widely found in elemental nanocrystals such as aluminum, iron, copper, gallium, and so on.78−80 However, the reported origins for the formation of 5-fold twinned structure are the effect of applied stresses, local internal shear stresses, thermally induced stresses during annealing. Figure 11a and b depict a landscape view of the potential energies of silver atoms and the distribution of average potential energies of atoms, respectively, pertaining to the hcp, fcc, bcc, ico, and other configurations. The distribution of average potential energy reveals that the fcc atoms have lowest potential energy compared to others. The trend pertaining to potential energy can be observed as fcc < hcp < bcc < ico < others, which may be attributed to nonhomogeneous distribution of packing density of silver atoms that are under the influence of carbon atoms of the SWCNT. 3.3. Dependence of Atomic Organization on SWCNT Diameter. In the previous section, it has been observed that the nonhomogeneous energy of silver atoms encapsulated within a SWCNT might lead to a combination of different atomic organisational configurations of the atoms as against a fcc organization in bulk. This contrast is attributed to the atomic interaction of the carbon atoms with those of the silver atoms up to several atomic distances into the nanowires. This implies that the size of the nanotubes may also influence the atomic organization of the silver atoms upon encapsulation. Therefore, in the present investigation, an attempt has been made to look for the organizational variation of the silver atoms inside SWCNT with different diameters. A wide range of SWCNT 20475


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Figure 12. Organization of silver atoms in a nanowire encapsulated into a SWCNT of various diameter: (a) 54.00, (b) 40.68, (c) 33.85, (d) 20.30, and (e) 10.85 Å. Red, blue, green, and yellow color atoms have been indicated hcp, bcc, fcc, and ico atoms. For clear visualization, SWCNT have not shown in snapshots.

along the axis of SWCNT of 20.30 Å diameter can be considered as a single silver atom chain surrounded by a six silver atom ring. Figure 16 shows the variation in the total energy of silver atoms, during cooling from 1500 to 10 K, encapsulated into SWCNTs of various diameters ranging from 10.85 to 54.00 Å. The figure clearly reveals that the total energy of silver atoms, encapsulated within SWCNT of diameter d = 10.85 and 13.56 Å, gradually decreases with a decrease in temperature. However, for SWCNT of diameter d = 20.30, 40.60, and 54.00 Å, initially energy gradually decreases during cooling from 1500 to 1300 ± 25 K. At temperature 1300 ± 25 K, energy of silver quickly decreases due to phase transition from liquid to crystalline. Further, as temperature below 1290 K, energy of silver atoms gradually decreases again. From energytemperature plot, it can be concluded that phase transition from liquid to crystalline have found for silver atom encapsulated into SWCNT of diameter d = 20.30, 40.60− 54.00 Å. On the contrary, a metastable phase evolves of silver

along the axis of SWCNT (Figure 14c). A total of 13 silver atoms in a 1−5−1−5−1 stacking sequence is defined as a unit cell of the silver nanowire. The overall pseudo-1D morphology of silver atoms along the central axis of SWCNT can be considered as a single silver atom chain surrounded by fivesilver atoms ring. Similarly, the organization of silver atoms encapsulated into SWCNT of diameter d = 20.30 Å has been depicted in Figure 15a−c. A full view of the encapsulated SWCNT is given in Figure 15a and b, with and without the covering by SWCNT, respectively. In contrast to the configuration observed for the encapsulated atoms in 10.85 Å diameter SWCNT, the atoms in 12.20 Å SWCNT organize themselves in a 6-fold symmetry along the axis of SWCNT. The 6-fold symmetry of silver atoms evolves due to two alternating atomic layers containing a single atom and a sixatom ring stacked together to form a 1−6−1−6−1 stacking structure along the axis of SWCNT. The unit cell of silver nanowire thus created contains 15 silver atoms in the 1−6−1− 6−1 sequence. The pseudo-1D morphology of silver atoms 20476


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Figure 13. Organization of silver atoms inside the SWCNT (d = 10.85 Å) during cooling from 1500 to 10 K: (a) snapshot of silver atoms at various temperatures and (b) variation in % of silver atoms with fcc, bcc, hcp, ico, and other structures with decreasing temperature. Yellow and gray color atoms depict the ico and other atoms. Red arrows indicate the sequence of snapshots during the cooling from 1500 to 10 K.

Figure 15. Silver atoms organization encapsulated into a SWCNT of diameter d = 12.20 Å: (a) snapshot of silver atom encapsulated into SWCNT, (b) magnified view of pseudo 1-D organization of encapsulated silver atoms, and (c) a unit cell consisting of 15 silver atoms organization as 1−6−1−6−1. Figure 14. Silver atoms organization into a SWCNT of diameter d = 10.85 Å: (a) snapshot of silver atom encapsulated into SWCNT, (b) magnified view of pseudo 1-D organization of encapsulated silver atoms, and (c) a unit cell consisting of 13 silver atoms organization as 1−5−1−5−1.

square displacement increases sharply in liquid state. However, as silver atoms solidify into crystalline state, mean square displacement become stagnant. It is confirmed from mean square displacement that silver atoms encapsulate in SWCNT of diameter d = 20.30, 40.60−54.00 Å exhibit phase transition from liquid to crystalline state. However, silver atom

atoms encapsulated into SWCNT of diameter d = 10.85−13.56 Å. Figure 17 shows the mean square displacement of silver atoms encapsulated into SWCNT of various diameter. Mean 20477


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• The phase transition from liquid to crystalline depends on the diameter of the SWCNT. At low diameter (d = 10.85 and 13.56 Å), a meta stable solid phase has been found. On the contrary, silver atoms encapsulated into SWCNT of diameter (d = 20.30 and 54.00 Å) have been transformed in liquid to crystalline phase. • In the low diameter case, silver atoms are kinetically trapped in the metastable state and unable to crystallize. However, in the large diameter case, silver atoms have found enough thermal mobility in the liquid state prior to crystallization, as confirmed from mean square displacement. The above results also imply that the SWCNT engendered the organization of silver nanowire to a significant extent. The presence of numerous type of nanostructural organization metal nanowire encapsulated into SWCNT will influence overall properties such as thermal and electrical conductivity along with mechanical properties. The catalytic activity, optical, electrochemical, biosensing, antibacterial properties of silver nanowire implicitly/explicitly relate to length/diameter ratio and structural organization of silver atoms. However, in the future, we will further investigate to relate length/diameter ratio and structural organization of silver nanowire (or nanostructure) with these properties.

Figure 16. Variation in potential energy of silver atoms encapsulated into a SWCNT of diameter ranging from d = 10.85−54.00 Å during cooling from 1500 to 10 K.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; [email protected]. ORCID

Sunil Kumar: 0000-0003-2276-3087 Notes

The authors declare no competing financial interest.

■

Figure 17. Mean square displacement of silver atoms encapsulated into a SWCNT of diameter ranging from d = 10.85−54.00 Å during cooling from 1500 to 10 K.

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4. CONCLUSIONS In this study, we have sought to understand the encapsulation and organization process of silver atoms into a SWCNT of diameter ranging from d = 10.85−54.00 Å by using molecular dynamics simulations. It has been found that the silver atoms encapsulate into a SWCNT in the liquid state. The interaction potential between silver and carbon atoms of SWCNT along with the intra-atomic potential of silver atoms are responsible for encapsulation processs. The organization of encapsulated silver atoms is found to be started at 1290 K during the cooling process (1500 to 10 K). The major finding pertaining to phase transition from liquid to crystalline phase are as given below: • A cylindrical pattern of crystalline phase of silver nanowire have evolved which consist of fcc, bcc, hcp, ico, and others atoms inside the SWCNT of diameter d = 40.60 Å. • A 5- and 6-fold symmetry of crystalline silver nanowire have been observed inside the SWCNT of diameter d = 10.85 and 12.20 Å. 20478


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