Fully Atomistic Molecular Dynamics Simulations of the Isothermal

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

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Fully Atomistic Molecular Dynamics Simulations of the Isothermal Orientation of n‑Decanes Confined between Graphene Sheets Hua Yang,*,† Yan Fang Liu,† and Hui Zhang‡ †

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Tianjin Key Laboratory of Structure and Performance for Functional Molecules, Key Laboratory of Inorganic-Organic Hybrid Functional Material Chemistry, Ministry of Education, College of Chemistry, Tianjin Normal University, Tianjin 300387, P. R. China. ‡ Key Laboratory of Engineering Dielectrics and Its Application of Ministry of Education & College of Chemical and Environmental Engineering, Harbin University of Science and Technology, Harbin 150080, People’s Republic of China S Supporting Information *

ABSTRACT: The molecular films as candidates for functional electronic materials has prompted numerous investigations of underlying mechanisms for their structure and formation. We present the results of extensive fully atomistic molecular dynamics simulations of the isothermal orientation of n-decanes confined between graphene sheets. At low temperatures, the ndecanes are normally oriented to the basal plane of graphene. With increase in temperature, the n-decanes are laterally oriented to the surface. When temperature is high enough, the n-decanes form a disordered structure (melting). The orientation-ordered parameters, end-to-end distance, interplanar distance of graphene sheets, and coordinates of nalkanes are calculated and used for describing the isothermal orientation processes at different temperatures, which can be considered as a threestep process (adsorption, orientation, and growth). n-Decane molecules adopt a twisting motion to change their positions and orientations. Simulation temperature changes the interactions between n-decane and graphene and the interactions between n-decane molecules. The interactions of n-decane−graphene and n-decane−n-decane govern the alignment of n-decanes confined between the graphene sheets.

1. INTRODUCTION Graphene, owing to its ultrahigh carrier mobility and oneatomic thickness, has been proposed as a promising material for next-generation electronic devices and fundamental studies of nanoconfined molecules. However, the freestanding graphene films are fragile and environmentally sensitive.1,2 Two-dimensional supramolecular assembly of n-alkane molecules between graphene surfaces is a method to control the spatial chemical and electronic properties of graphene.3 With the orientated n-alkanes on graphene, carrier mobilities are enhanced by several times and the environmental sensitivity is greatly reduced. Different thermodynamic and kinetic variables can also be used to pattern the formation of orientated nanomaterials.4 Thus, it is important to know the mechanism of formation of molecular order and orientation for n-alkanes at the nanoscale in different conditions. The behavior of n-alkanes confined on or between graphene surface(s) has been an intriguing subject for understanding the functions of chains materials. When n-alkanes are adsorbed onto the surface such as graphene, graphite, they generally assemble lamellae structure.3−17 Masnadi and Urquhart5,6 found n-alkane chains laterally oriented at high supercooling temperature and normally oriented at the smallest supercooling temperature when n-alkane thin films were vapor © XXXX American Chemical Society

deposited on a highly oriented pyrolytic graphite surface. nDecane adopted a parallel orientation between parallel graphene surfaces.7 The adsorbed single polyethylene chain was orientationally linked to graphene as a two-dimensional folded chain crystallite.8 The crystal growth of n-alkanes on tetrahedrally coordinated crystals was studied by Bourque et al.9,10 The isotropic−nematic transition of semiflexible polymer solutions confined between two parallel walls was simulated using GPU-accelerated Langevin dynamics.11 Svatek and his co-workers3 found that the adsorbed ordered alkanes can also induce deformation of the graphene substrate. The phase transitions of ordered layers of alkanes on graphite surface were also simulated and characterized.12−17 The systems change toward disordered states with increasing temperature. The n-alkane chains are usually laid parallel or perpendicular to the basal plane and aligned regularly with van der Waals interactions. The mechanism of the ordered structure formation is defined by the mono- and multilayer formation and the structural evolution of the ordered structures as a Received: June 28, 2018 Revised: September 19, 2018 Published: October 19, 2018 A

DOI: 10.1021/acs.jpcc.8b06191 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

MD simulations of 3000 ps at high temperature (T = 600 K) are performed to confirm that the n-alkane molecules have relaxed completely. The initial configuration of 120 n-decane molecules between the graphene sheets is presented in Figure 1 and denoted by G-120C10.

function of temperature. The orientation is controlled by the substrate temperature experimentally and effected by the substrate property (slippery or sticky walls) computationally. For alkane and polymer, lateral orientation or edge-on lamellae predominantly developed at low substrate temperature and faster evaporation rate, whereas normal orientation or flat-on lamellae predominantly formed at high substrate temperature and slower evaporation rate.5,6,18−22 Monte Carlo simulation predicted that thin films with slippery walls exhibit a dominantly edge-on lamellae at high temperatures, whereas thin films with sticky walls show mainly a flat-on lamellae.23,24 Confined alkanes have been extensively studied in the past via molecular dynamics (MD) simulation.9,10,25−29 The surface nucleation and crystal growth for n-alkane on foreign surfaces have been described and the effect of nucleating agents has also been identified by MD simulation.9,10 The volumetric, structural, and conformational properties of melt polyethylene/graphite and melt polyethylene/vacuum interfaces have been shown directly.26,27 The influence of patterned graphite surface on the adsorption and crystallization of polyethylene has been investigated from a microscopic point of view.28 Kalyanasundaram et al.29 showed the organization and behavior of n-decane confined between two gold {111} surfaces with fixed distance. Much essential information has been provided at the molecular level. Although these studies extended our insights into the orientation of chain molecules confined on the surface. Some common but very important information on the orientation of confined short-chain molecules has not been clearly described. A comprehensive understanding of the orientation of the short-chain molecule between two surfaces is still lacking because most previous investigations did not focus on the film in nanoscale and the effect of the fluid temperature. In this study, the MD simulation is employed to investigate the orientation of n-decane (C10) between graphene sheets at different temperatures. The two two-dimensional surfaces are one-atom layers and the distance between them can change with the variation in the n-decane molecules during our MD simulations. Our simulation results can be used to study the effects of temperature on the orientation of n-decane molecules confined between ultrathin surfaces. The mechanism of ordered structure formation of the n-decane molecules melts confined between graphene sheets will be clarified at the molecular level.

Figure 1. Initial configurations for 120 n-decane molecules confined between graphene sheets. The C10 molecules are denoted in different colors.

Our simulations use the COMPASS force field,30−32 which has been successfully used to simulate the interaction between alkane/polyethylene and the carbon nanotube/graphite/ graphene.24,33−35 Energy calculations with COMPASS are a combination of valence and nonbonding terms. The valence terms include stretching, bending, and torsion energies as well as the diagonal and off-diagonal cross-coupling terms. The nonbonding terms are the van der Waals and electrostatic interactions, which are calculated with a cutoff Rcut = 12.5 Å and a spline width 1 Å. The isothermal orientation of the ndecane molecules between the graphene sheets is studied. The 10 000 ps NVT MD simulations are performed for G-120C10 at temperatures between 300 and 600 K with 25 K interval. The equations of motion are integrated with a time step of 0.001 ps. Simulation temperatures are maintained using the Nosé−Hoover thermostat.36−38 As the graphene sheets are exposed to vacuum, the distance between them will change when the temperature changes. The total density of our models does not change throughout our NVT MD simulation. But, the density of 120C10 confined between the graphene sheets will change as the temperature changes.

2. COMPUTATIONAL DETAILS The system studied in this paper is that of n-alkane molecules adsorbed between graphene sheets, which are exposed to vacuum. At the beginning, we build a single-crystal cell of graphite with the cell parameters a = b = 2.46 Å, c = 6.80 Å, α = β = 90°, and γ = 120°. The graphene sheets in our simulation are obtained from 20 × 20 supercells. The periodical boundaries are applied to the systems. To ignore the interactions between the adsorbed n-decane and the periodic images of graphene sheets in c direction, the cell parameter c is enlarged to 60 Å. The three-dimensional periodicity is transformed into actual two-dimensional periodicity by this way. Then, 120C10 molecules are built and placed randomly within a simulation box. After that, they are put into the middle of two parallel graphene sheets. Hydrogen atoms are explicitly included in the models. These C10 molecules can form a single ordered layer with their backbones perpendicular to the graphene sheets and have 100% coverage on the surface. The

3. RESULTS AND DISCUSSION 3.1. Process of Isothermal Orientation of G-120C10. MD simulations can provided an insight into the ordered structure formation of alkane chains. Our simulations indicate that n-decane chains will have lateral orientation at a relative high temperature and a normal orientation at low temperature. The two typical isothermal relaxation processes for G-120C10 at 450 and 350 K are shown in Figure 2a,b, respectively. The C10 chains are denoted in different colors. At the beginning of the simulations, the C10 chains are adsorbed onto the graphene sheets and disordered in the alkane layer. With the simulation proceeding, local orientation appears at about 500 and 1200 ps when the simulation temperatures are 450 and 350 K. After that, some of the C10 molecules move into the ordered region(s) slowly and the orientation region become greater gradually. This demonstrates a three-step isothermal orientation process: adsorption, orientation, and growth. The B


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Figure 2. Isothermal relaxation process for G-120C10 at 450 K (a) and 350 K (b). The C10 molecules are denoted in different colors.

the other C10 chains are adsorbed onto one graphene sheet and parallel to the surface; some C10 chains enter the ordered structure and the regularities of the adsorbed layer and the normal orientation structure increase after the ordered structure is formed. The conformations of 2000, 5000, and 10 000 ps show these obviously in Figure 2a,b. 3.2. Order Parameter of n-Decane Layers. To quantify and characterize the structure of 120C10 during the isothermal orientation process, order parameters are calculated to monitor molecular orientation. We first calculated the bond-orientation order parameter OPb,39 which is defined by

C10 chains need about 800 ps MD simulations to orient laterally to the graphene sheet at 450 K and about 1500 ps MD simulations to orient normally to the graphene sheet at 350 K. The alkane fluid relaxes slowly at lower temperature. Thus, the appearance and formation of orientation in the alkane layer at 350 K are much later than that at 450 K. For G-120C10 at 450 K, almost all the C10 chains are parallel to the graphene sheet; most of them form two lateral ordered structures between the disordered chains; the chains in the ordered structure are also parallel to each other; there are five C10 layers between the graphene sheets in the zdirection; some C10 chains enter the ordered regions and the regularity of the structure increases after the ordered structure is formed. For G-120C10 at 350 K, most of the C10 chains are perpendicular to the graphene sheet, and they are also parallel to each other; only one normal ordered structure is formed;

OPb = C

3 cos2 ϕ − 1 2

(1) DOI: 10.1021/acs.jpcc.8b06191 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C where ϕ is the angle between the subbond vector and the zaxis, and the subbond vector is formed by connecting centers of two adjacent bonds. The parameter OPb would assume a value of 1.0, 0.0, or −0.5, respectively, for the alkane molecules whose subbonds are perfectly parallel, random, or perpendicular to the z-axis. There are five ordered C10 layers in the last ordered structure for G-120C10 at 450 K. Therefore, the 120C10 are divided into five layers. The OPb of the five layers are calculated for G-120C10 at 450 and 350 K and shown in Figure 3. The three-step isothermal orientation process is also

of C10 are parallel to and the others are perpendicular to the zaxis in layers 1 and 5; thus, OPb of layers 1 and 5 are around 0.0 after ordered structure formation. Checking Figure 3a,b carefully, OPb of layers 2, 3, and 4 decrease earlier at 450 K and increase earlier at 350 K than that of layers 1 and 5 as the result of a relatively weak interaction between the graphene sheets and the C10 in the three layers. To investigate the orientation of C10, global orientation order parameter g-OPb of all the C10 is calculated by the equation g‐OPb =

3 cos2 φ − 1 2

(2)

where φ is the angle between two neighboring subbond vectors. The parameter g-OPb would assume a value of 1.0 for the alkane molecules are extended and perfectly parallel to each other. Figure 4 shows the time evolution of the global orientation order parameter g-OPb and the bond-orientation order

Figure 4. Time evolution of global orientation order parameter g-OPb and bond-orientation order parameter OPb of G-120C10 at 350 and 450 K.

parameter OPb of G-120C10 at 350 and 450 K. g-OPb quickly increases to 0.88 and OPb quickly decreases to −0.4 before 800 ps at 450 K. This shows that the 120C10 transit to a bondorientation ordered structure and the subbonds of C10 are perpendicular to the z-axis at 450 K. For G-120C10 at 350 K, g-OPb increases to 0.94 slowly and OPb increases to 0.6 quickly from 1000 to 4000 ps. Another kind of bond-orientation ordered structure is formed where the subbonds of C10 are parallel to the z-axis at 350 K. This also shows a three-step isothermal orientation process. The C10 chains between two ordered regions are disordered in the last conformation of G120C10 at 450 K (Figure 2a) and almost all the C10 are extended conformation in the last conformation of G-120C10 at 350 K (Figure 2b). Therefore, g-OPb of G-120C10 is bigger at 350 K than that at 450 K. Both g-OPb and OPb of G-120C10 also change earlier at 450 K than at 350 K. 3.3. Configurations of n-Decane and Graphene. The orientation information on 120C10 between two graphene sheets is shown in the previous section. The structures of C10 and graphene sheets will be described in this section. The endto-end distance (Red) can show the conformation of a short alkane backbone directly. The extended n-alkane and curved n-

Figure 3. Time evolutions of bond-orientation order parameters OPb of five layers in G-120C10 at 450 K (a) and 350 K (b).

described in Figure 3. At the beginning of the simulations, OPb of layers 1 and 5 are about −0.2 and OPb of layers 2, 3, and 4 are about 0.0; when C10 are near the surfaces, some of them are parallel to the graphene sheets; C10 molecules are disordered when they are in the middle of the 120C10 (layers 2, 3, and 4). At 450 K, OPb of the five layers decreases as the time increases; they drop quickly from 500 to 800 ps and all fluctuate around −0.4 after 1000 ps. The five ordered layers form and are perpendicular to the z-axis. At 350 K, OPb of layers 1, 2, 3, and 4 increase slowly before 1000 ps and then increase rapidly; at last, OPb of layer 1 fluctuates around 0.2 and OPb of layer 2, 3, and 4 fluctuate around 0.725, 0.875, and 0.800, respectively; C10 in the three layers are parallel to the zaxis; OPb of layer 5 decreases to −0.4 before 3000 ps and increases to 0.0 quickly after that. Figure 2b shows that some D


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orientation begins to form at this simulation stage. After that, the interplanar distances decrease quickly again and fluctuate around 23.7 Å at 450 K and around 22.3 Å at 350 K. Ordered orientation structure forms at this stage. As the C10 molecules are mostly extended conformation in an ordered structure, the ordered structure takes less space than the disordered structure. The interplanar distances decrease when the ordered structure appears. The perpendicular ordered structure takes less space (350 K) than the parallel ordered structure (450 K). The times of end-to-end distance and interplanar distance changes are consistent with those of the orientation order parameter changes. During the orientation of C10 chains between graphene sheets, the C10 molecules change from gauche to extended conformation and the interplanar distance decreases; ordered regions appear and enlarge. Let us check the last conformations of G-120C10 after 10 000 ps MD simulations at 350 and 450 K. The side view in Figure 2a and the top view in Figure 2b show that both ordered structures have hexagonal symmetry, and the stems (C10) can be not only perpendicular but also parallel to the graphene sheets. These results are similar to the crystal of polyethylene, branched polyethylene, and their compositions with graphite.40−42 To describe the conformation of n-decane, the C−C−C−C dihedral angled of n-decane for G-120C10 at 450 K are calculated and the dihedral angle distributions within the different layers averaging from 9001 to 10 000 ps are shown in Figure 7. There are two kinds of peaks in the

alkane adopt an extended conformation and a gauche conformation. Thus, Red of extended C10 chain will be larger than that of curved C10 chain. The time evolution of average Red of 120C10 at 350 and 450 K is shown in Figure 5. The Red

Figure 5. Time evolution of the average end-to-end distance of C10 at 350 and 450 K.

changes little before 200 ps at 350 K and before 300 ps at 450 K. After that, Red increases from 9.7 to 11.5 Å slowly and fluctuates at last at 350 K. At 450 K, Red increases from 9.5 to 11.0 Å quickly from 300 to 800 ps and fluctuates around 11.0 Å after that. As Red increases, the C10 molecules change from gauche conformation to extended conformation gradually, and ordered orientation regions appear and grow. This also shows that the appearance and formation of the orientation in the ndecane layer at 450 K are much quicker than that at 350 K. The interplanar distance between the graphene sheets is also calculated and shown in Figure 6. The initial model of our

Figure 7. C−C−C−C dihedral angle distribution of n-decane within the different layers for G-120C10 from 9001 to 10 000 ps at 450 K. The inset is an enlarged part of the last conformation (10 000 ps).

distribution. The first one is C−C−C−C dihedral angles between 30 and 100° or −30 and −100°. The peak indicates the presence of the gauche conformation of n-decane. The second one is the dihedral angle between 150 and 180° or −150 and −180°, which corresponds to the trans conformation of n-decane. The second peak is much higher than the first one. This means almost all the C10 adopt a trans conformation. The sequence of the height of peaks of the five layers at ±180° is layer 1 ≈ layer 5 > layer 2 ≈ layer 4 > layer 3. This results from the effect of the graphene sheets. The interaction between them decreases with increasing distance between them. An enlargement of the last conformation of G120C10 at 450 K is also shown in the inset of Figure 7. The C10 molecules are parallel to each other in the ordered region.

Figure 6. Time evolution of the interplanar distance between graphene sheets at 350 and 450 K.

simulations is obtained at 600 K. Therefore, the interplanar distances decrease quickly at the beginning for G-120C10 at 350 and 450 K. The chain conformations (Red of 120C10) are not affected by the graphene sheets at this stage. Then, the interplanar distances both change little from 200 to 2000 ps at 350 K and from 300 to 500 ps at 450 K. The induced chain E


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increase with decrease in the interplanar distance at the two temperatures. A continuous density distribution of n-decane forms at 350 K and a five-peak distribution appears at 450 K after that. At 350 K, the heights of the two high peaks decrease and then more CHxs shift to the middle of the C10 layer. And at 450 K, the heights of the five peaks barely change through the rest of the simulation time. It should be noted that the first and fifth peaks are higher than the second and forth ones, which are also higher than the third one. This clearly describes the influence of graphene on the C10 molecules. The local bond orientation order of the C10 molecules along the z-axis can be described by the local bond orientation ordered parameter SB(z), which is defined by

A C10 molecule is in an all-trans configuration to adsorb, with its carbon skeletal plane parallel to the graphene sheets and an outer plane of hydrogen atoms occupying the centers of graphene carbon hexagons. This is consistent with the structural model for n-alkane monolayer adsorbed on the graphite surface.12,43 3.4. Local Properties of G-120C10. To show more details of the isothermal orientation process, we try to calculate the local properties of the studied system. The local mass density of n-decane is calculated at different positions (z) for G-120C10 at 350 and 450 K and shown in Figure 8. CHx has a

SB(z) =

3 cos2(ϕ(z)) − 1 2

bond

(3)

where ϕ(z) is the angle between the subbond vector and z-axis and ⟨···⟩bond denotes the average over the subbods in a slab between z and z + dz. We set dz = 0.5 Å when calculating SB(z). The SB(z) would assume a value of 1.0, 0.0, or −0.5, respectively, for the subbonds in a slab perfectly parallel, random, or perpendicular to the z-axis. The SB(z) of the configurations at different simulation times for G-120C10 at 350 and 450 K are provided (Figure S1 in the Supporting Information). We can see a continuous ordered structure is formed during the MD simulation at 350 K. The C10 molecules are parallel to the z-axis in the ordered structure. In the case of 450 K, five ordered layers gradually form and some microstructures appear in the five layers during the MD simulation. The process is similar to the crystallization of polyethylene on the graphite surface.37 The subbonds or segments of the C10 molecules are perpendicular to the z-axis in the five layers. The G-120C10 forms normal orientation at 350 K and lateral orientation at 450 K. The subbonds or segments of the C10 molecules are perpendicular to the z-axis near the graphene sheets at the two temperatures. This also shows the influence of graphene on the adsorbed C10 molecules. 3.5. Shift of C10 Molecules During the Orientation Process. In a crystal, atom, molecule, or chain segment vibrates at or near specific locations. In this section, we will explore the shift behavior of C10 molecules during the isothermal orientation process of G-120C10 and discuss the difference for C10 at 350 and 450 K. The motion of C10 will be described in two directions, which are normal (zcoordinate) or parallel (x-coordinate) to the graphene surface. Six typical C10 molecules (denoted by A, B, C, D, E, and F) are chosen to display the shift behavior of the C10 molecules during the orientation process. We provide the time evolution of six C10 molecules’ center of mass during the 10 000 ps MD simulations at 350 and 450 K (Figures S2 and S3 in the Supporting Information). Our MD simulations show that the orientational C10 molecules are parallel or perpendicular to the z-axis at 350 and 450 K, respectively. At 350 K, the z-coordinate of C10 A changes throughout the 10 000 ps MD simulation; the zcoordinates of the other five C10 molecules change clearly, for C10 B before 3500 ps, for C10 C before 2400 ps, for C10 D before 1800 ps, for C10 E before 4200 ps and for C10 F before 2000 ps; and all fluctuate around a value after that except C10 E. The z-coordinate of C10 E changes twice from 4200 to 7000 ps and fluctuates around a value after 7000 ps. In other

Figure 8. Local mass density distribution for G-120C10 at different simulation times during the orientation process at 350 K (a) and 450 K (b). The positions of the graphene sheets are marked as two small peaks at the edge of the density distribution.

wide distribution when the relaxation begins. There are two high peaks near the graphene sheets throughout the simulations, which means that more C10 molecules or segments are adsorbed near the graphene sheets. This is similar to the density distribution of C10 under nanoconfinement between the gold surface.29 As the simulation begins, the distributions become narrow and the heights of the peaks F


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Figure 9. Last conformations of G-120C10 after 10 000 ps NVT MD simulation at different temperatures: (a) side view and (b) top view. The C10 molecules are denoted in different colors.

words, the C10 molecules shift obviously between the graphene sheets in the z-direction before aligning to the ordered region and some molecules can move a short distance in the z-direction after that at a relatively low temperature. At 450 K, the z-coordinates of C10 C, D, and E increase or decrease quickly before 800 ps and fluctuate around a value after 800 ps; the z-coordinates of C10 A, B, and F all change obviously throughout the MD simulation. This means that the C10 molecules (C, D, and E) can shift before they align to the ordered regions and vibrate in the z-direction after that; the C10 molecules (A, B, and F) located between the ordered regions can shift between the graphene sheets at a relative high temperature. It can be seen that the changes in the xcoordinate of the six C10 molecules are the same at 350 K and similar at 450 K. This shows that the C10 molecules move as a whole at 350 or 450 K in the x-direction. Our simulations give an anisotropy motion of C10 during an isothermal orientation between the graphene sheets. The C10 molecules move obviously in the direction perpendicular to the graphene surface (z) before they align to the ordered regions. For G-120C10 forming a lateral ordered structure at 450 K, the C10 molecules in the disordered regions shift between the graphene sheets and the C10 molecules vibrate in the ordered

regions after they align into the ordered regions; for G-120C10 forming a normal ordered structure at 350 K, the C10 molecules do not change their position or shift a little after the ordered structure is formed. All the C10 molecules as a whole move parallel to the graphene surface (x) at 350 and 450 K. To describe the shift process of the C10 molecules during the isothermal orientation, the end-to-end distances and the bond-orientation order parameters of the six C10 molecules are also calculated. Red can describe the conformations (bending or extended) of the studied alkane. Red of extended C10 lies between 11 and 12 Å. OPb can show the orientation of alkane chain to the z-axis. The time evolutions of Red and the bond-orientation order parameters (OPb) of the above six C10 molecules at 350 and 450 K are also provided (Figures S4 and S5 in the Supporting Information). At 350 K, OPb of C10 A is about −0.4 at most of the simulation time, which means C10 A has a lateral orientation to the surface. OPb of the other five C10 molecules all increase from negative values to about 0.9, which means that they adopt a twisting motion until they are orientated normally to the graphene sheets at 350 K. They enter ordered regions at different simulation times, C10 B at about 3500 ps, C10 C at about 2400 ps, C10 D at about 1800 ps, C10 E at about 4200 G


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The Journal of Physical Chemistry C ps, and C10 F at about 1600 ps. Combining the data of Red with OPb, the C10 molecules (B, C, D, E, and F) become extended chains at the same time as they enter the ordered region. Red of C10 A is less than 11 Å at most of the simulation times at 350 K. These means that C10 A is not a completely extended chain during the MD simulation. Red and OPb change synchronously with the their z-coordinate. At 450 K, Red of C10 A, B, and F fluctuate in a much wide variation range during the 10 000 ps MD simulations, describing their quick and heavy twisting motion; OPb of C10 A, B, and F also change in a wide variation range (−0.4− 0.9) during the 10 000 ps MD simulations; Red and OPb of the other three C10 molecules are both changeable before 800 ps and their Red fluctuates around extended chain length, their OPb fluctuates around −0.4; based on the above analysis of the orientation processes at 450 K, the three moving C10 molecules (A, B, and F) keep on twisting to shift between the graphene sheets and their orientations also change correspondingly; the other three C10 molecules adopt a twisting motion before they orientate parallel to the graphene sheets at 450 K. Combining the above discussions, we can conclude that the C10 molecules adopt a twisting motion to change their positions and orientations. 3.6. Isothermal Orientation of G-120C10 at Different Temperatures. We also investigate the effect of temperature on the orientations of 120C10 confined between the graphene sheets. Figure 9 gives the last conformations of G-120C10 after 10 000 ps NVT MD simulation at different temperatures. The 120C10 molecules can form ordered structure confined between the graphene sheets when the temperature is below 475 K; on the other hand, a disordered structure is formed when the temperature is higher than 500 K. The lateraloriented structures are observed at 375, 400, 425, 450, and 475 K. The long-axis of the C10 molecules align parallel to the graphene surface. The normal-oriented structures are obtained at 325 and 350 K. The long-axis of the C10 molecules align, in an all-trans configuration, normal to the graphene surface. We can find lateral-oriented and normal-oriented regions in the last configuration at 300 K. To show the effects of temperature quantitatively, we run an average of OPb and g-OPb of all the C10 molecules from 9001 to 10 000 ps at each simulation temperature. Figure 10 shows changes in the average OPb and average gOPb for 120C10 molecules with increasing temperature. g-OPb is about 0.9 from 300 to 425 K, decreases from 425 K, and is about 0.6 after 500 K. This denotes that the C10 molecules have an extended conformation before 475 K and bend to gauche conformation after 500 K. OPb of −0.18 at 300 K is a result of the coexistence of lateral orientation and normal orientation of C10 and about 0.6 at 325 and 350 K as a result of the normal orientation of C10 to the graphene sheet. OPb is about −0.5 from 375 to 425 K, increases from 425 K, and is about −0.18 after 500 K. This indicates that the C10 molecules are parallel to the graphene sheets at 375, 400, 425, 450, and 475 K and disordered after 500 K. The increase in OPb and decrease in g-OPb from 475 to 500 K show the transition from order to disorder. Simulation temperature is important in the alignment of C10 between the graphene sheets. Normal-oriented C10 molecules will experience a graphene interaction via the methyl (−CH3) group, whereas lateraloriented C10 molecules experience a graphene interaction via the entire length of the C10 chain. The interactions between the C10 molecules exist in both two orientation models.5 The

Figure 10. Average of OPb and g-OPb for all the C10 molecules in G120C10 vs temperature. The error bars are the standard deviation calculated from the average (9001−10 000 ps).

orientation of the C10 molecules between graphene sheets reflects the delicate balance between C10−C10 interactions and C10−graphene interactions. Figure 11 provides the

Figure 11. Interaction energies of C10−graphene and C10−C10 for G-120C10 at different simulation temperatures.

interaction energies of C10−graphene (Ecg) and C10−C10 (Ecc) for G-120C10 at different simulation temperatures. Ecc for G-120C10 is much lower than Ecg when the temperature is lower than 475 K. The C10 molecules orient and form an ordered structure between the graphene sheets. When the temperature is higher than 500 K, Ecc for G-120C10 is almost as much as Ecg. The C10 molecules form a disordered structure. When the C10 molecules align normally to the graphene sheets at 325 and 350 K, Ecg is larger than when they align laterally to the graphene sheets. And Ecc at 325 and 350 K is much lower than that at other simulation temperatures. This means that only when the C10−C10 interaction is relatively strong enough can they align normally to the graphene sheets. Our results is similar to Wang’s results.28 As the simulation temperature changes, Ecg and Ecc change correspondingly and the C10 molecules form different orientation structures. H


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4. CONCLUSIONS We have discussed here the isothermal orientation process of 120 n-decane molecules confined between graphene sheets at different temperatures. Our calculations reveal the details of the nanoscale mechanism of lateral and normal orientation formations among the simulation temperatures. The applied molecular dynamics simulations allow us to display the two processes directly at atomic level. It has been found that G120C10 forms a lateral-oriented ordered structure at relatively high temperatures and a normal-oriented ordered structure at relatively low temperatures. In summary, the isothermal orientation process can be considered as a three-step (adsorption, orientation, and growth) process. The evolution is quicker at high temperature than at low temperature. In the first step, the C10 molecules are adsorbed onto the graphene surfaces; different layers appear gradually, whose orientation changes little; the C10 chains bend and shift their position in the z-direction violently. The second step takes a relative short time than the third step; the orientation of the C10 molecules increases quickly and ordered regions (lateral or normal oriented) appear correspondingly; most of the C10 molecules adopt an extended conformation and their end-to-end distances increase; the interplanar distance between the graphene sheets decreases; at the end of this step, the C10 molecules in the ordered regions will not shift in the z-direction clearly. In the third step, the orientation of the C10 molecules increases further; some of the C10 molecules that are not in the ordered regions can move in the z-direction; most of the properties of the systems change little. The C10 molecules adopt a twisting motion to change their positions and orientations. Simulation temperature is important in the alignment of C10 between graphene sheets. The ordered structure of G-120C10 is governed by the interactions of C10−graphene and C10−C10. As simulation temperature changes, the interactions change correspondingly. G-120C10 will be normal and lateral orientation coexistence, normal orientation, lateral orientation, and disordered structure at different simulation temperatures. Only when the C10−C10 interaction is relatively strong enough can they align normally to the graphene sheets. When the interaction of C10−C10 is almost as much as the interaction of C10− graphene, G-120C10 forms a disordered structure. Our simulations give valuable insights into the mechanism of the lateral and normal orientation formations of the n-decane layer confined between the graphene sheets at different simulation temperatures, which is difficult to reveal experimentally, specially at atomic-scale description for the interaction. The interaction is important in understanding the orientation of n-alkane molecules confined between graphene sheets. More research is needed to further investigate the influence of thin film thickness on the orientation in the future.

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end-to-end distances (Figure S4), and bond-orientation order parameters (Figure S5) (PDF)

AUTHOR INFORMATION

Corresponding Author

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

Hua Yang: 0000-0001-7508-3435 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the Program for Innovative Research Team in University of Tianjin (grant number TD13−5074) and National Science Foundation of Tianjin (No. 18JCYBJC21800). Financial supports were provided by young and middle-aged teacher of Tianjin Normal University (52XC1201).

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REFERENCES

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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.8b06191. Local bond orientation ordered parameter of the configurations at different simulation times (Figure S1), time evolution of six typical C10 (A, B, C, D, E, and F) molecules’ center of mass (Figures S2 and S3), I


Article

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Fully Atomistic Molecular Dynamics Simulations of the Isothermal

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