Wettability and Structural Evolution of Gold over a Single-walled

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C: Physical Processes in Nanomaterials and Nanostructures

Wettability and Structural Evolution of Gold over a Singlewalled Carbon Nanotube: an Atomistic Investigation Sunil Kumar J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b02885 • Publication Date (Web): 25 Jun 2018 Downloaded from http://pubs.acs.org on June 25, 2018

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The Journal of Physical Chemistry

Wettability and Structural Evolution of Gold over a Single-walled Carbon Nanotube: an Atomistic Investigation Sunil Kumar CSIR-National Metallurgical Laboratory, Jamshedpur, India 831007 Email: [email protected], [email protected]

Abstract Gold nano-structures with high surface area to volume ratio depicts many applications for the design and development of advance materials for the nano-elecronic, catalyst, optoelectronic, anti-bacterial, etc. In order to extensive applications, gold nano-structures can be synthesized by the deposition over various substrate, such as carbon nanotube, graphite, silica, etc. In the present study, a thin molten gold film has been deposited over a single walled carbon nanotube (SWCNT) to observe the wettability and phase transitions using molecular dynamics simulation. At high temperature (T = 2000 K), gold film over a SWCNT depicts poor wettability and evolves into a globule. However, during cooling from 2000K to 10K, gold globule depicts phase transition from liquid to face centered cubic (fcc) crystalline structure. At interface between gold and SWCNT, gold atoms organized both on- and off-positions over hexagonal arrangement of carbon atoms of SWCNT. In case of on-position, gold atom positioned in the middle of the hexagonal arrangement of carbon atoms. Off-position gold atom situated just above of the C-C bond of SWCNT. Solvent accessible surface areas (S), solid volume (V), dimension-less aspect ratio ( =

), contact angle, adaptive common neighbour analysis, and

radial density distribution function have been used for the analysis of wetting and structural evolution of gold over SWCNT. The present theoretical investigation will enhance our understanding related to the solid state physical phenomena for the development of various type of metallic nano-structures.

Key words: Gold Nanostructure, SWCNT, Molecular Dynamics Simulations, Interface, Adsorption, Encapsulation. 1 ACS Paragon Plus Environment

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1.

Introduction Wetting behaviour of metallic film over the carbon nanotube have attracted widespread

interest due to their extraordinary mechanical, optical, electronic, and catalytic properties1-5. Silver, gold, platinum, copper, aluminium, etc. can be adsorbed over the cylindrical substrate of single wall carbon nanotube (SWCNT), which open the new possibility to develop film with extremely high aspect ratio6-16. Various experimental17-21 and theoretical22-32 investigations have been carried out for the synthesis and characterization of metallic film in the vicinity of SWCNT as reported in the litrature. Ajayan17 developed an ex-situ experimental procedure for the encapsulation of various materials into the multi-walled carbon nanotubes (MWCNT) for the development of metallic nanowires, with an encapsulation yield of up to 90%. Later, Ajayan and Ebbesen18 confirmed that this approach can be successfully applied to fill up MWCNTs without preliminary opening of its ends. Segura et al.19 have depicted that gold nanoparticle in the vicinity of carbon nanotube depict unique five fold symmetry in the crystalline phase by experimental methodology. Hsu et al.21 have developes highly efficient experimental methodology for the encapsulation and adsorption of gold nano-clusters over thiolated carbon nanotube through thiol-metal bonding. Despite of various experimental investigations of metallic nano-cluster adsorption and encapsulation in the vicinity of carbon nanotube, some theorectical mathods also implemented by various research groups to study the wettability and phase trasition of metallic material over the carbon nanotube. Molecular dynamics simulation technique has been extensibly used to investigate various properties of metallic materials over the carbon nanotube substarte as reported in the litrature22-32. Chen et al.22 have studied the melting behaviour of silver nanowire with 5-fold twinned boundaries of various diameters using molecular dynamics simulations. They found that the melting temperature of silver nanowire decreases with the decreasing diameter. Arcidiacono et al.23 have studied the wettability and solidification of gold nano-cluster inside

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the SWCNT. They have revealed that the solidification temperature of gold nano-cluster depends on their size and diameter of SWCNT. Rezaei et al.24 have investigated the mechanical property of copper-zinc (Cu-Zr) alloy along with the carbon nanotube (CNT) by molecular dynamics simulations. They found that the long CNT significantly increased the stiffness and yield strength of the Cu-Zr alloy. In contrast, metallic glasses reinforced with short CNT depicted almost no improvement of mechanical properties. Cui et al25-27 have demonstrated nanoscale

soldering/welding

procedure

during

adsorption/enacapsulation

of

metallic

nanoparticle over the axially positioned SWCNT by molecular dynamics simulations. In our prevous study28-32, we have investigated the structural evolution (phase transition) of materials in the vicinity of graphene and SWCNT during solidifications using molecular dynamics simulation. The inclusion of graphene and SWCNT in the materials considerably influence the phase transitions and properties of its composites. Despite of these studies, the issues related to the wetability and evolution of various structures of gold nano-cluster over cylindrical substrate of SWCNT remain unanswered. The present study is an attempt to have an insight into the wettability and phase transition of gold nano-cluster over the SWCNT using molecular dynamics simulation. The simulation system consists of the multiple steps: such as estimation of wettability of gold film over a SWCNT at 2000 K, phase transition of gold over SWCNT from 2000 K to 10 K (cooling rate 0.1 K/ps). The radial distribution function, contact angle, adaptive common neighbour analysis, solvent accessible surface area, solid volume, dimensionless aspect ratio have been used to characterize the wettability and phase transition of gold over the SWCNT. To the best of the author’s knowledge, such broad investigations addressing a multitude of coupled phenomenological issues such as wettability and evolution various structures of gold over the SWCNT have not been attempted earlier in the light of published literature.



2.

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Simulation details Constant temperature molecular dynamics simulation33,

34

has been implemented to

perform simulations for the wettability and phase transition of gold film over a SWCNT. A generalized Finnis and Sinclair form of embedded atom method potential35, 36 used among the gold atoms. The total energy of gold atoms, is given by

1 = V − (1) 2

! = " (2) #

! = " (3) $

where is the separarion between & '( and ) '( gold atoms, and ! energy and lattice

parameter for the gold atoms. is the pair potential for the repulsion between & '( and ) '( gold atoms.

The inter-atomic potential energy between gold and carbon atoms of SWCNT have been carried out through 12-6 Lennard Jones potential as given by the equation (4). *+ = 4 - ./

01234 78 5

6

−/

01234 9 5

6 :

(4)

Where - , and ; - are the 12-6 Lennard Jones parameters for minimum energy and equilibrium interatomic distance at null potential respectively between gold and carbon atom. The numerical values of - = 0.01> to 0.05> and ; - = 2.9943 Å have been

used for inter-atomic interaction between gold and graphene substrate23,37,38. The values of σ and ε depend over the materials characteristics as determined by experimentally37 for the metal and carbonaceous nano-filler interfaces such as SWCNT and graphene. The velocity Verlet algorithm39 with a time step of 2 femto-second (fs) has been used to solve Newton’s equation of motion to update position and velocity of each gold atom during molecular dynamics 4 ACS Paragon Plus Environment

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simulation. The Nose-Hoover thermostat40,41 has been implemented to maintain the temperature and cooling rate of gold-SWCNT composite system during molecular dynamics simulations. Open source softwares such as Large Scale Atomic Molecular Massively Parallel Simulator42, Visual Molecular Dynamics43 and Open Visualization Tool44,45 have been used for molecular dynamics simulations, visualization and analysis of output simulation data of gold – SWCNT system. The simulation system consists of a cylindrical film of randomly oriented gold atoms over a single wall carbon nanotube of diameter, d = 40Å, as shown in Figure 1(a) - (c). Simulation system has been designed in three steps. In the first step, 100000 gold atoms equilibrated in bulk at 2000K for 1000 pico-second to make a homogeneous liquid. In the second step, a hollow cylindrical film of gold section cut from the bulk. In the third step, a single wall carbon nanotube (SWCNT) placed inside the hollow cylindrical gold film as shown in Figure 1(a)-(c). After the design of the simulation system, molecular dynamics simulations performed to study the wettability and phase transition of gold over SWCNT. During solidification (phase transition), temperature of the gold decreases from 2000 K to 10 K at a cooling rate of 0.1 K/ps. In contrast to the reported literature46-54, somewhat slower cooling rate has been adopted in the present investigation so as to efficiently capture the evolution of the nanocrystalline structure of gold over the SWCNT. SWCNT kept rigid during wetting and solidification of gold film during adsorption and solidification process. It is illustrated in the literature52-59, that the rigid nano-substrate does not affect the wettabilty of the metallic material, but it reduces the computational expense drastically.

3.

Results and Discussion The results obtained from the present attempt on molecular dynamics simulations of

wettability and solidification of gold film over a SWCNT have been presented and discussed.



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This contains various aspect of the wettability, evolution of various structures in gold over a SWCNT.

3.1.

Wettability of gold film over a SWCNT A cylindrical gold film has been placed over a SWCNT, to study their wettability using

molecular dynamics simulation. Figure 2 depicts the snapshots of temporal evolution of wetting/dewetting process of gold over the SWCNT at various time during simulation at fixed temperature T = 2000K. In the early stage of simulation (up to 100 ps), gold depict dewetting and form a ring like structure over the SWCNT, however as simulation proceeds ring like structure transformed into globule and minimize wetting area with the SWCNT. The radial distributions of gold (Au) atoms with respect to carbon atoms of SWCNT calculated through D12 〉, where, N and V are the function B- (C)60, 61 as defined by B- (C) = D 〈∑K7 HIG ∆G

12

#(G)

the total number of gold atoms and volume of the system, respectively. O(C) is depicting the number of gold atoms in a spherical shell of radius χ and thickness ∆χ from the carbon atom of

SWCNT. The radial density distributions, B- (C), of gold atoms over SWCNT at various time during dewetting process have shown in supporting information (Figure S1). At 1ps, the

value of B- (C) found maximum due to spreading of gold over the SWCNT. With the increase of simulation time (i.e. 100ps, 500ps and 1000ps), the values of B- (C) decreases

due to dewetting of gold (or formation of gold globule) over the SWCNT. Solvent accessible surface area (S), solid volume (V) and deimension-less aspect ratio ( =

P

) of gold over SWCNT have been estimated during simulations. In order to estimate S

and V, a mesh has been created over the gold to depict as solevent accessible surface as shown in Figure 3. The algorithm for the construction of surface mesh over the gold is based on the alpha-shape method as developed by Edelsbrunner and Mucke62. It carried out by choosing a Delaunay tetrahedrization over the surface atoms. The algorithm for the construction of surface 6 ACS Paragon Plus Environment

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mesh and calculation of its surface area is inbuilt in open source software OVITO44,45. The probe radius r = 2.85Å, used to compute S and V. During dewetting process, solvent accessible surface area (S) and dimension-less aspect ratio () decreases due to aggregation of gold atom over the SWCNT. On the contrary, solid volume (V) of gold increases as shape of gold change from thin film to globule over the SWCNT as shown in Figure 4. In general, SWCNT exhibit poor wettabilty by molten gold. Poor wetting generally leads to weak interfacial bonding between gold and SWCNT. The extent of wetting of gold

over SWCNT can be modelled by tuning the 12-6 LJ potential energy parameter (εAu-C) as shown in Figure 5 (a) – (d). At εAu-C = 0.01eV, the gold depicts very poor wetting and make a globule over the SWCNT. With the increasing value of εAu-C, gold globule spread over the SWCNT. High values of εAu-C, (~ 0.04eV to 0.05eV) indicate strong wettability between the gold and SWCNT. This indicates the interaction energy (εAu-C) controls the wettabilty between

gold and SWCNT. With the increasing value of - , the shape of gold globule over the

SWCNT have found globule, ring and cylindrical type. Globule shape of gold depicts very less wetting area over the SWCNT, compared to ring and cylindrical shape. Supporting information Figure S2 depicts radial density distributions gAu-C(χ) of gold atoms over the

SWCNT at various values of - . As the values of - increases from 0.01eV to 0.05eV, the value of gAu-C(χ) also increase. Increasing values of gAu-C(χ), depict good wetting of gold

over the SWCNT substrate. These increasing values of gAu-C(χ) with χ at various values of

- indicates the wettability significantly influenced by interfacial interations. Variation in S, V and =

P

of gold over SWCNT have been estimated at various values of - as

shown in Figure 6. The values of S, V and increases with increase in - due to high wettability. During the cooling process, the temperature of gold decreases from 2000 K to 10 K. The snapshots show the evolution of various structures of gold atoms, such as face-centered 7 ACS Paragon Plus Environment


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cubic (fcc), body-centered cubic (bcc), and hexagonal closed packed (hcp), over the SWCNT as snapshots shown in Figure 7. At high temperature (T > 1075K), gold live in liquid phase. However, at temperature below T < 1075K, gold depicts phase transition over SWCNT, which evolves in step-wise manners as liquid to metastable bcc to stable fcc (with hcp) structures. It is according to the Ostwald’s step rule, in which gold depicts phase transitions in step-wise, i.e. metastable bcc structure evolves from liquid and then changes into stable fcc structures. Metastable bcc structure of gold have shown relatively higher energy and volume per atom compared to the stable fcc structures. The radial distribution function of gold atoms,

B (C), has been estimated during cooling process as shown in Figure 8. At high

temperatures, i.e. 2000 K to 1090K, a single peak of B (C) have evolved only at C ≈

2.85 Å, due to a short range ordered structure of gold atoms over SWCNT. However, as

temperature in the range (1090K to 1030K), the multiple peaks of B (C) gradualy evolved, which indicate the nucleation and crystal growth in gold. Temperature below 1030K,

multiple peaks of B (C), has been distinctly evolved to depict both short and long range

ordered structure in gold. First peak of B (C), represents 1st nearest neighbour for the gold

atoms at C ≈ 2.86Å, which is almost equal to ! ×

√8 ≈ 2.88 Å, where 8

! is the lattice parameter

of gold (! = 4.078 Å). These explanation indicates that the gold over the SWCNT,

demonstrate both short and long range ordered structure. Figure 9 shows the percentage evolution of gold atoms with fcc, bcc, hcp and ico atoms during the cooling from 2000 K to 10 K. It is found that, at high temperatures (T > 1090 K), most of gold atoms are in liquid state which is organized randomly. Further, with decrease in temperature below (T < 1090 K), a phase transition from liquid to solid (crystalline) in gold over SWCNT is observed. The bcc structure is the most favourable intermediate organization of gold atoms during phase transition. Similar bcc structural evolution of metallic materials has also been reported by Desgranges and Delhommelle55 for the crystallization of pure metal in bulk. At low 8 ACS Paragon Plus Environment

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temperatures i.e. T < 300 K, most of the gold atoms crystallize themselves in a stable combination of fcc with hcp twin plane over SWCNT. The crystallization of gold depicts fcc structure with twin plane of hcp over the SWCNT as shown in figure 10 (a) –(c). The direction of twin plane of hcp gold atoms depends over the - . At - = 0.01>, gold evolved in a globular shape over the SWCNT with

very low wetting over SWCNT. Gold crystallize in fcc crystal with hcp twin plane. However, the direction of twin planes have not been any correlation with the SWCNT axis

as shown in Figure 10(a). As the value of - increases the orientation of hcp twin plane align along the axis of SWCNT as shown in Figure 10 (b)-(d). The hcp twin plane along the axis of SWCNT helps to gold atom to crystallize in fcc structure with maximum possible compatibility and wettability with the SWCNT. Two types of positional arrangements preferred by gold atoms over a SWCNT, which are on- and off-organization as shown in Figure 11. In case of on-organization, gold atoms positioned just above middle of the hexagonal arrangements of carbon atoms of SWCNT. In case of off-organization, gold atoms located above the carbon-carbon covalent bonds of SWCNT. The gold atoms in on-organization over SWCNT is in lower potential energy state compared to off-organizations. Figure 12 depicts the variations in the total energy of gold over SWCNT at various values of εAu-C, (0.01eV to 0.05eV) during cooling from T = 2000 to 10 K. At T = 1442 K (for εAu-C, = 0.05eV), 1330 K (εAu-C, = 0.04eV), 1210 K (εAu-C, = 0.03eV), 1030 K (εAu-C, = 0.02eV), and 990 K (εAu-C, = 0.01eV), total energy of gold quickly decreases due to phase transition from liquid to crystalline. It confirmed that the temperature for the phase transition of gold from liquid to crystalline over the SWCNT have found to be dependent on the value of εAu-C. As the value of εAg-C increases, the corresponding temperature for the phase transition also increases. The wetability of gold over the SWCNT has been quantitatively estimated by contact angle (θ). Contact angle calculated as W = cos7 /− 6, where, R and h are the radius and [ (



height of fitted circle over the gold, as shown in Figure 13 (a). Parameter, ℎ =

_=

]^ `] 8]^

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]^ ] 8]^

and

were calculated by the height of the globule, x1, and the half of the cross-sectional

interface length, x2, over the SWCNT as similarly reported in ref63-66. Hence, contact angle (θ) can be calculated using relation, W = cos7 /

] ]^ ] `]^^

6 for the gold adsorption over the SWCNT at

various values of - . Figure 13 (b) shows the calculated values of contact angles (θ) of gold over the SWCNT, which have found to be strongly depends over the interfacial non-

bonded interactions between gold and SWCNT ( - ). At - = 0.01> and 0.02eV, contact angle of gold over SWCNT found to be θ =144.60 and 1120, which depicts very poor

wetablity. However, the contact angle decreases to θ = 600 for the - = 0.04>, which

depict enhance wetting over the SWCNT. Heyraud and Metois67 have estimated contact angle, θ = 126.90 ± 0.20 for gold crystallites over the graphite substrate experimentally, which strongly depends over the nature of contact plane. Habenicht et al.68 have found contact angle, θ = 1310, of gold over the graphitic substrate. With few limitations of molecular dynamics simulations methodology and its parameterization, the contact angle of gold over SWCNT at - =

0.01> and 0.02eV have found fairly consistant with experimental values as reported in ref.6768

.

3.2. Discussion. The results associated with the wetting and phase transition of metallic material over the CNT provide understanding for enhancing the dispersion in the metal matrix which is existing issue till now. Physical and chemical treatment of CNT for the enhance wetability of metal is a very efficient way to synthesis of catalyst for various applications such as electrochemical reactions, supercapacitors and other energy storage devices. For these


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applications, very high specific solvent accessible surface area essential for extremely quick chemical reactions with high conversion rate of desired product. Simulation results depict the physical treatment of gold-SWCNT interface can be modelled by tunning the van der Waals interactions ( - ). Tunning of Van der Waals interactions facilitate to change the physical characteristic of interface and processing conditions (like effect of solvency)56-58 to generalize the considered model. The evolution of various structures in gold over SWCNT at different values of Van der Waals interactions ( - ) and temperatures have studied. High interaction value - = 0.04>, shows good wettability and high surface area at a wide range

processing temperature. On the contrary, lower interactions depict very poor wettability with SWCNT which deteriorates interface and composite properties. The results associated with the organization of gold atoms at interface between goldSWCNT (on- and off organizations) greatly influence the various properties of metal-matrix nanocomposites. Li et al.59 have found similar organization of aluminium (Al) over the graphene flakes by experimentally. The on- and off-organization of metal atoms at interface controls the crystal nucleations, stress, permeability, electric and heat transfer capabilities during real applications. The inclusion of CNT, graphene and other carbonaceous nano-filler in metal matrix also influences the physical properties of material such as melting temperature, glass formation ability, grain size etc. Therefore, the wettability and phase transition of metallic materials at interface is desired for further scientific understanding.

4.

Conclusions In this study, it has investigated the wetting and phase transition of gold onto a SWCNT

using molecular dynamics simulations. It has been found that the gold depict poor wettability

into a SWCNT. The interaction potential ( - ) between gold and carbon atoms of SWCNT is responsible for wettability. At low interaction potential, gold depict very poor wettability and



evolve into a globular structure over the SWCNT. On the contrary, as interaction potential inceases, wettability of gold enhances over the SWCNT. During solidification, crystallization of gold have found to be started at 1090 K during the cooling process (2000 K to 10 K) onto a

SWCNT. The interaction potentials ( - ) also influences the orientations of fcc and twin plane of hcp atom of gold in crystalline phase. Two types of positional arrangements by gold atoms over a SWCNT have been found such as on- and off-organization. In case of onorganization, gold atoms positioned middle of the hexagonal arrangements of carbon atoms of SWCNT. In case of off-organization, gold atoms positioned above the carbon-carbon covalent bonds of SWCNT. The above results implies that the SWCNT induced organization of ultrathin gold film will influence overall properties such as thermal and electrical conductivity along with catalytic, anti-microbial, mechanical etc. However, in future, we will further investigate to relate length/diameter ratio and structural organization of gold nano-structure with the properties.

Supporting Information The radial distribution functions, gAu-C(χ), of gold over the SWCNT have been depicted in the supporting information. Figure S1 shows the extent of wetting by gAu-C(χ) vs. χ plots at various time during simulation. Further, gAu-C(χ) vs. χ plot (Figure S2) depicts the effect of various values of εAu-C, onto the wettability of gold over the SWCNT.

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Page 20 of 33

130Å

Side view

Side view

(a) 10Å

Top view (b)

Top view (c)

Figure 1 Preparation of simulation system for the study of wettability and phase transition of gold over the SWCNT: (a) 100000 gold atoms in the liquid phase, (b) side and top view of hollow cylindrical gold film section cut from the bulk, and (c) a single wall carbon nanotube (SWCNT) of diameter (d = 40Å) placed inside the cylindrical gold film. The length of gold film is 130Å and thickness 10Å.

ACS Paragon Plus Environment

Page 21 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 40ps 28 Initial system 29 30 31Figure 2 Side and top views of wetting 32temperature T=2000 K. Red color arrows 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


100ps

500ps

1000ps

behavior of gold film over a SWCNT with increasing time and at a fixed are indicating the progress of dewetting process.


The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 Initial system 40ps 100ps 500ps 28 29 Figure 3 Surface mesh created over the gold for the estimation of solvent accessible surface area (S) 30 31during dewetting process at T = 2000K. For the clear visualization, SWCNT have not been shown. 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 ACS Paragon Plus Environment 60

Page 22 of 33

1000ps and solid volume

Page 23 of 33

6000

 = S /V

2

Dimension-less aspect ratio () 3

3000

0

Solvent Accessible Surface Area (S)

02

S (A )

48000 36000 24000

200000

Solid Volume (V)

03

V (A )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


180000

160000 0

250

500

750

1000

Time (ps)

Figure 4 Variation in solid volume (V), solvent accessible surface area (S), and dimension-less aspect ratio (𝜅 =

𝑆3 𝑉2

) , of gold during dewetting over SWCNT with time at fixed temperature

T=2000K.



(a)

(b)

Page 24 of 33

(c)

(d)

Figure 5 Snapshots of side and top view of wetting of gold over a SWCNT, with increasing value of interactions between gold atom to SWCNT at 2000 K: (a) 𝜀𝐴𝑢 −𝐶 = 0.01 𝑒𝑉, (b) 𝜀𝐴𝑢 −𝐶 = 0.02 𝑒𝑉, (c) 𝜀𝐴𝑢 −𝐶 = 0.03 𝑒𝑉 , and (d) 𝜀𝐴𝑢 −𝐶 = 0.04 𝑒𝑉.


Page 25 of 33

3000

3

 = S /V

2

Dimension-less aspect ratio () 1500

0

Solvent Accessible Surface Area (S) 02

S (A )

45000

30000

15000 220000

Solid Volume (V)

03

V(A )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


210000

200000 0.00

0.01

0.02

Au-C

0.03

0.04

0.05

(eV)

Figure 6 Variation in solid volume (V), solvent accessible surface area (S), dimension-less aspect ratio (𝜅 =

𝑆3 𝑉2

) , of gold over SWCNT at various values of 𝜀𝐴𝑢 −𝐶 at temperature T=300K.



Page 26 of 33

2000K 1075K 1050K 300K Figure 7 Snapshots of phase transition from liquid to crystalline state of gold over SWCNT during cooling from 2000 to 10 K. Blue, red, and green color gold atoms pertain to body-centered cubic (bcc), hexagonal closed packed (hcp), and face-centered cubic (fcc), crystal structures respectively. For clear visualization, gold atoms with random organization have been shown in small dots.


Page 27 of 33

1200 st

0

1 Nearest Neaghbour at  = 2.86A

Arbitrary unit

900

gAu-Au()

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


600

Crystalline phase (fcc)

300

Liquid to crystalline phase transition Liquid phase

0 3

6

9

12

10K 100K 200K 300K 400K 500K 600K 700K 800K 900K 1000K 1030K 1050K 1060K 1070K 1075K 1085K 1090K 1100K 1200K 1300K 1400K 1500K 1600K 1700K 1800K 1900K 2000K

15

0

 (A ) Figure 8 Radial distribution functions, 𝑔𝐴𝑢 −𝐴𝑢 𝜒 , of gold during cooling from 2000 to 10 K. The separation between first nearest neighbors of gold atoms is indicated by arrows. Liquid, transition and crystalline phase are shown by black, red, and green color lines at respective temperatures.



100

Others 50 0

ico

0.03

0.00

% of atoms

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 33

12

hcp

6 0 60

fcc 30

Liquid phase

Crystalline phase

0

bcc

20

0 2000

1600

1200

800

400

0

Temperature (K)

Figure 9 Percentage evolution of gold atoms with fcc, hcp, bcc, and ico structures over a SWCNT during cooling from 2000 to 10 K.


Page 29 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


𝜀𝐴𝑢 −𝐶 = 0.01eV




(a)

(b)

(c)

Figure 10 Organization of gold atoms over a SWCNT at various values of 𝜀𝐴𝑢 −𝐶 : (a) side view of fcc, and hcp atoms, (b) twin plane structure of hcp atoms and (c) top view. For clear visualization fcc and other atoms have not been shown in (b) and (c). Red, blue, green, and yellow color atoms indicate hcp, bcc, fcc, and ico atoms.



On- organization

Off- organization

Figure 11 Organization of gold atoms over the gold-SWCNT interface. Green and pink color atoms depict off- and on- organization over the hexagonal arrangements of carbon atoms of SWCNT respectively.


Page 30 of 33

Page 31 of 33

-3.0 Liquid phase

Au-C = 0.01eV Au-C = 0.02eV

Total Energy (eV/atom)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Au-C = 0.03eV

-3.2

Au-C = 0.04eV Au-C = 0.05eV

-3.4

-3.6

T = 1442K

Phase transition

T = 1330K T = 1210K T = 1030K T = 990K

Crystalline phase

-3.8 2000

1500

1000

500

0

Temperature (K) Figure 12 Variation in total energy of gold at different values of εAu-C during cooling from 2000K to 10K.



Side view

Side view

Schematic diagram for the estimation of contact angle Top view

Slice cut sections Top view (a) 180 150

Contact angle (Degree)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 33

120 90 60 30 0 0.00

0.01

0.02

u-C

0.03

0.04

0.05

(eV)

(b) Figure 13 Estimation of contact angle of gold over the SWCNT: (a) schematic for the estimation of contact angle and (b) calculated values of contact angles.


Page 33 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


[For table of contents only] Dewetting

Initial system (2000K)

Solidification

1000ps (2000K)

1075K

300K

Blue, red and green color atoms pertain to bcc, hcp and fcc structure, respectively


Wettability and Structural Evolution of Gold over a Single-walled

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