Understanding Liquid–Solid-Like Behavior of Tetrahydrofuran

Sep 30, 2013 - Department of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125, United States ... Nev...
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Understanding Liquid−Solid-Like Behavior of Tetrahydrofuran Adlayers at Room Temperature between Graphene and Mica: A Born−Oppenheimer Molecular Dynamics Study Shuang Chen,† Hui Li,‡ Peigen Cao,§ and Xiao Cheng Zeng*,† †

Department of Chemistry and Nebraska Center for Materials and Nanoscience, University of Nebraska-Lincoln, Lincoln, Nebraska 68588, United States ‡ Institute of Physics, Chinese Academy of Sciences, Beijing 100080, People’s Republic of China § Department of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125, United States S Supporting Information *

ABSTRACT: The phase behavior of adlayers of volatile small organic molecule tetrahydrofuran (THF) at room temperature has been imaged using atomic force microscopy (AFM) through graphene templating (J. Am. Chem. Soc. 2011, 133, 2334−2337). To gain more insight into the dynamical and structural properties of THF adlayers on mica and the effect of graphene templating, the Born− Oppenheimer molecular dynamics simulations (with the BLYP-D functional and a Gaussian plane-wave basis set) are performed. Without the graphene coating, the computed self-diffusion coefficients of THF molecules in the monolayer and bilayer are comparable to that in bulk THF solvent. However, with the graphene coating, the THF monolayer becomes considerably viscous. As the thickness of adlayers increases, the second adlayer of the THF bilayer exhibits even solid-like behavior, consistent with the AFM measurement. Although the motion of THF molecules becomes markedly slower with the graphene coating, the adsorbed THF molecules can still freely tilt and rotate on the mica substrate. Hence, the graphene-coated THF monolayer and bilayer are not strictly as ordered as the monoclinic THF crystal. Nevertheless, with the graphene coating, some THF molecules in the monolayer and bilayer entail certain degrees of crystalline packing as the graphene coating serves as an energy barrier to prevent the volatile THF molecules from evaporation, thereby limiting their motion in both lateral and vertical directions.

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using a scanning tunneling microscopy (STM) tip.11 The graphene coating prevents evaporation of small molecules via limiting their motion. The water adlayers confined between mica(001) and graphene coating have been studied by Cao, Heath, and co-workers using AFM, and the water monolayer and bilayer possess the same thicknesses as a single layer and double layers of Ih-ice, respectively, indicating a robust ice structure at ambient conditions.6 In their subsequent experiment, Cao, Heath, and co-workers found that the lowmolecular-weight tetrahydrofuran (THF), a liquid with low viscosity at standard temperature and pressure, exhibits not only a layer-by-layer growth mechanism on mica but also mixed liquid and solid properties with the graphene coating.7 However, detailed dynamical properties and microscopic structures of the graphene-coated THF adlayers, as well as the effect of the graphene, require further studies. With the development of powerful supercomputers and highperformance quantum mechanics software packages, the highly demanding first-principles simulations become feasible for such

INTRODUCTION Graphene, a single layer of sp2-bonded carbon atoms in a honeycomb lattice, has been proven to be a remarkable coating material due to its excellent mechanical stability, low chemical reactivity, flexibility, impermeability to most gases, transparency, and high thermal and electrical conductivity.1,2 With the graphene coating, for example, the Pt(100) surface can be protected against O2 oxidation.3 Meanwhile, CO molecules can intercalate under the graphene layer when the CO pressure is higher than 10−6 mbar.3,4 For further oxidation of CO molecules, the graphene sheet can also serve as an imaging agent.4 Two laminated graphene layers are typically employed to trap droplets of Pt-containing solutions for Pt growth, and as such, the Pt nanocrystal nucleation process can be visualized at atomic-resolution level using the transmission electron microscopy (TEM).5 For highly diffusive water6 or volatile organic molecules,7 the graphene coating can trap thin layers of water or volatile molecules on the mica surface for atomic force microscopy (AFM) characterization of their structures under ambient conditions. Indeed, based on the graphene templating, water adlayers between a graphene and various substrates have been studied, including their microscopic structures,8 doping effects,9,10 as well as nanomanipulation of the water adlayers by © 2013 American Chemical Society

Received: August 27, 2013 Revised: September 29, 2013 Published: September 30, 2013 21894

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program implemented in the CP2K software package.16 The Becke−Lee−Yang−Parr (BLYP) functional17,18 is employed to compute the potential gradient. The core electrons are described by the Goedecker−Teter−Hutter (GTH) normconserving pseudopotential,19,20 and the wave functions of valence electrons are expressed by the combination of the polarized double-ξ quality Gaussian basis21 and a plane-wave basis set (with an energy cutoff of 280 Ry). To better describe the long-range electron correlations that are responsible for the van der Waals (vdW) interactions, the Grimme dispersion corrected (DFT-D) method22 is employed in the BOMD simulations. Following the geometry optimization, BOMD simulations are performed in the constant-volume and constant-temperature (NVT) ensemble to investigate properties of THF adlayers on the mica at room temperature (295 K). The entire simulation time for each model system is about 30.0 ps with the time step of 1.0 fs. The first 5-ps runs are used for equilibration, and the remaining 25 ps dynamic trajectories are used for the statistical analysis. To study the dynamical properties of THF adlayers and further depict the effect of graphene coating, the self-diffusion coefficients (D) of THF molecules in the monolayer and bilayer without and with the graphene coating are computed from the mean square displacement (MSD) as a function of time (t), based on the Einstein relationship, i.e.,

studies. In a previous simulation study, we employed the Born− Oppenheimer molecular dynamics (BOMD) simulations within the framework of density functional theory (DFT) to confirm the experimentally observed ice-like structure6 of water adlayers between the graphene coating and mica(001) substrate at ambient conditions.12 The water bilayer under the graphene coating exhibits high stability, resulting in the transition from a solid−liquid-like monolayer to a solid-like bilayer.12 In this simulation study, we focus on the phase behavior of volatile organic molecules, THFs, confined between the graphene coating and the mica substrate at room temperature. Since the THF molecules interact much more weakly with the mica surface than water molecules7 and they possess more complicated molecular structures, the theoretical investigation of graphene-coated THF molecules is more challenging. Through the BOMD simulations, dynamical and structural properties of THF adlayers on mica substrate, without and with the graphene coating, are obtained and compared with the experimental measurement.7

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COMPUTATIONAL DETAILS In the previous BOMD simulations of water adlayers on the muscovite mica (001) surface, we noted that the model system with K+ ions located between the ice film and the mica surface was less stable than that without K+ ions.12 For the THF adlayers on mica, the similar periodic models are built with THF molecules deposited on the top side of the mica (001) layer, where the K+ ions are located on the bottom side. Considering the high computational cost of a BOMD simulation, the slab dimensions are set to be 15.6 Å × 9.0 Å × 38.0 Å with a 3 × 1 unit cell in the lateral (x−y) dimensions and a single aluminosilicate layer in the z direction of the monoclinic crystal (C2/c2M1) of the muscovite mica (chemical formula: KAl2(Si3Al)O10(OH)2).13 Remarkably, in the AFM experiment, Cao, Heath, and colleagues found that the THF adlayers exhibit both liquid and solid properties, and the statistical height of the second THF adlayer agrees with the layer thickness of the monoclinic THF crystal.7 Thus, in the model system considered here, the THF bilayer (10 molecules) with the monoclinic crystal structure14 is deposited on the mica surface as the initial configuration for the BOMD runs (see Figure 1). This film is then coated by a graphene monolayer. For comparison, the graphene-coated crystal-like THF monolayer, and the monolayer and bilayer without the graphene coating are also built (Figure 1). The BOMD simulations within the framework of the Kohn−Sham formulation of DFT and with the Gaussian plane-wave (GPW) basis15 are carried out by using the QUICKSTEP

D=

⟨|r(t + t0) − r(t0)|2 ⟩ 1 lim 2d t →∞ t

(1)

where r(t0) represents position of THF molecule at the initial time t0 and d denotes the dimensionality of the thin films. The variations of MSD with time and corresponding fitting curves can be seen in Figure S1 of the Supporting Information.

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RESULTS AND DISCUSSION A. Dynamical Properties of THF Adlayers: SelfDiffusion Coefficients. As demonstrated in the experiment,7 the graphene coating can seal volatile molecules onto the atomically flat mica surface. Without the graphene coating, the THF molecules can escape from the mica surface at room temperature. In Table 1, the computed self-diffusion coefficients (D) of THF molecules are given. Without the graphene coating, little change in D is seen between monolayer and the second adlayer of the THF bilayer. Both D values are on the order of 10−5 cm2/s, close to typical self-diffusion coefficient of bulk liquids at room temperature (e.g., 2.84 × 10−5 cm2/s at 25 °C for bulk THF).23 With the graphene coating, the self-diffusion coefficient of THF molecules in both the monolayer and bilayer is reduced to the order of 10−6 cm2/ s. More importantly, the MSD of the second adlayer of the graphene-coated bilayer levels off beyond 7 ps (see Figure S1b, Supporting Information), implying a solid-like behavior for the second THF adlayer. Since the self-diffusion coefficients of monolayer and bilayer with the graphene coating are smaller than those without the coating, we can conclude that with the graphene coating, the monolayer as well as the first adlayer of the bilayer exhibit viscous liquid-like diffusive properties, that is, their diffusivity is less than that of bulk THF solvent at room temperature. These theoretical results are consistent with the experimental observation that the first THF adlayer is liquidlike, whereas the second THF adlayer is solid-like under graphene templating.7

Figure 1. Initial configurations of monolayer and bilayer without and with the graphene coating (blue bar), respectively, for the BOMD simulations. 21895

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Table 1. Computed Self-Diffusion Coefficients (Unit: cm2/s) of THF Molecules on Mica Surface without and with Graphene Coating through Linear Fitting of MSD-Time Curves Shown in Figure S1, Supporting Information, on Basis of BOMD Simulations bilayer

graphene-coated bilayer

monolayer

first

second

graphene-coated monolayer

first

2.4 × 10−5

4.4 × 10−6

2.0 × 10−5

4.5 × 10−6

4.4 × 10−6

second

Figure 2. Variation of height (H) of the graphene sheet with the BOMD time for graphene-coated mica (left), graphene-coated THF monolayer (middle), and bilayer on mica surface (right), respectively. In the insets, 30 ps snapshots of these three models are shown. At certain moments, the height of graphene sheet is averaged with respect to vertical height of each C atom within the graphene sheet. Thicknesses (ΔH) of the monolayer ΔH1 (= H1 − H, where H1 is the height of the graphene sheet on the THF monolayer) and second adlayers ΔH2 (assuming the thickness of the first adlayer in the THF bilayer is the same as ΔH1) are finally estimated according to the difference of the height profiles. For the thickness estimation, the height of the graphene sheet is further averaged over time. The layer thickness of the monoclinic THF crystal along b axis14 is also present for comparison.

adlayers are presented in Figure 2, where the averaged thickness of the monolayer is ΔH1 ≈ 4.67 Å, slightly thicker than that of monoclinic THF crystal in the b direction (4.47 Å).14 Here ΔH1 = H1 − H, where H1 is the height of the graphene sheet on the THF monolayer and H is the height of the graphene on mica. Assuming the thickness of the first adlayer in the THF bilayer is the same as ΔH1, the averaged thickness of the second adlayer would be ΔH2 ≈ 4.11 Å, slightly thinner than that of the first adlayer ΔH1. In the experiment,7 the measured thickness of the second adlayer (ΔHexp 2 ≈ 4.4 Å) is slightly larger than that of the monolayer (ΔHexp 1 ≈ 4.2 Å). The difference between computed thicknesses and the measured ones is likely due to the difference in mica surfaces between the theoretical model and realistic one in the experiment (i.e., weaker THF/mica interaction than the experimental one). As a consequence, the simulated graphene-coated THF monolayer is more diffusive than the measured one, thereby the thickness of the THF monolayer (ΔH1 ≈ 4.7 Å) (see Figure 2) is larger than that of measured 7 monolayer (ΔHexp 1 ≈ 4.2 Å), even larger than that of crystalline monoclinic THF crystal in the b direction (4.47 Å).14 In our BOMD simulations, the probability distribution of the adlayer thickness, h, evaluated from the vertical distance from the top of the molecule to the top of the mica surface, is further analyzed, as shown in Figure 3. The definition of h differs from the definition ΔH mentioned above. Here, h can be extended to cases without the graphene coating. The h-distribution of THF monolayer without the graphene coating ranges from 4− 8.7 Å. Zero value in h-distribution is seen at h = 4.47 Å, which corresponds to the thickness of a crystalline layer. For the THF bilayer without the graphene coating, the peak of the hdistribution for the first adlayer shifts to a lower h than that of the monolayer without the graphene coating due to the

On the basis of the simulation trajectories (Supporting Information Movies S1−S4) and computed D, we find that the packing structures become increasing ordered from the monolayer and bilayer without the graphene coating, then to monolayer with the graphene coating, and finally to bilayer with the graphene coating, consistent with the observed dynamical behavior of the THF adlayers, i.e., from liquid with low viscosity, then to viscous liquid with high viscosity, and finally to mixed liquid-solid-like bilayer at room temperature. In other words, the graphene coating not only can prevent the evaporation of volatile THF molecules but also can alter their dynamical properties and phase behavior. B. Structural Properties of THF Adlayers. Adlayer Thickness. In the previous AFM experiment, the height of adlayers is an important parameter to assess possible solid-like packing structures for the THF adlayers.7 Cao, Heath, and coworkers found that the measured heights of the first (4.2 ± 0.2 Å) and second (4.4 ± 0.2 Å) adlayers are in agreement with the layer thickness of the monoclinic THF crystal along the b axis, although the first adlayer is ∼5% thinner.7 To compare with the measured heights from the AFM experiment, the height of the graphene sheet from the mica surface is statistically analyzed. As a reference, we first analyze the height of graphene coating from mica, followed by the height of graphene-coated monolayer and bilayer on mica, respectively, on the basis of the BOMD trajectories from 5 to 30 ps (see Figure 2). The height (H) is a mean distance from the graphene sheet to the bottom of the mica substrate. The thicknesses (ΔH) of the monolayer and second adlayers can be computed from the differences in height profiles between graphene-coated mica, graphene-coated monolayer on mica, and graphene-coated bilayer on mica (this definition of thickness ΔH was used in the experimental measurement7). The time-dependent thicknesses of the 21896

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Figure 3. Probability distribution of adlayer thickness (h) of the monolayer and bilayer without and with graphene coating, respectively. Here, the adlayer thickness is defined as vertical height from the top of the molecule to the top of the mica surface. The blue vertical lines highlight the values of one-layer and two-layer thickness, respectively, of monoclinic THF crystal along the b axis.14

Figure 4. Probability distribution of orientation angle (θ) of monolayer and bilayer without and with graphene coating, respectively. The molecular axis of THF is defined as the vector from the center of the C−C bond to O atom in THF ring. Then, the orientation angle (θ) of THF molecules is estimated as the azimuthal angle of molecular axis to the z direction (perpendicular to the mica surface). Two blue columns in insets indicate the orientation of THF molecules with the perfect crystal packing.14 The upward and downward orientations of THF molecules are divided by the dark cyan line with an orientation angle of 90°.

confinement by the second adlayer. With the graphene coating, both peaks of the h-distribution become sharper. The peak value for the graphene-coated monolayer is h1 ≈ 4.8 Å (close to ΔH1 ≈ 4.7 Å), and the two peak values of the graphene-coated bilayer are h1ad ≈ 5.2 Å and h2 ≈ 9.2 Å, respectively. Note that in our simulation, h1ad ≈ 5.2 Å > h1 ≈ 4.8 Å ≅ ΔH1 ≈ 4.7 Å, so the thickness of the first adlayer in the bilayer is slightly larger than that of the monolayer (see Figure 3) due to the higher diffusivity of molecules in the first adlayer than those in the monolayer. Note also that the peak of h-distribution for the second adlayer is much sharper than that for the first adlayer, indicating that under the graphene coating, molecules in the second adlayer is much less diffusive than those in the first adlayer (consistent with results shown in Table 1). The peak values of graphene-coated monolayer h1 ≈ 4.8 Å and bilayer h2 ≈ 9.2 Å show that the corresponding thickness is only slightly greater than the thickness of a single crystalline layer (4.47 Å) and two crystalline layers (8.94 Å), respectively. In addition, these peak values are notably smaller than the peak values corresponding to monolayer and the second adlayer of bilayer without the graphene coating, indicating that the graphene coating limits significantly the vertical motion of THF molecules, thereby the molecules can pack closely to the substrate. To describe structures of the confined THF layers in more detail, the molecular orientation (Figure 4), atomic density profiles (Figure 5), and intermolecular translation and rotation (Figure 6) are investigated. Orientation of THF Molecules. As shown in the inset of Figure 4, the molecular axis of a THF molecule can be defined as the vector from the center of the C−C bond to O atom. As such, the orientation angle (θ) of the THF molecule can be defined as the azimuthal angle of molecular axis with respective to the z direction. If the THF adlayers on mica surface are assumed perfect monoclinic crystal packing as the initial configurations shown in Figure 1, the THF molecules would stand upright along the z direction, and half the number of molecular vectors are pointing away from the substrate (θ = 0°), while the other half point toward the substrate (θ = 180°). In the BOMD simulations, the upward and downward orientations of the THF molecules can be distinguished by θ < 90° and θ > 90°. As shown in Figure 4, the θ distribution for the monolayer and bilayer without and with the graphene

coating are not densely populate at 0° or 180°, suggesting that the THF molecules generally tilt on the mica surface. For the THF monolayer without the graphene coating, θ peaks up at about 70°, indicating that most THF molecules tend to lie down on the mica substrate. The θ distribution of the THF bilayer without the graphene coating exhibits three peaks, at 25−37°, 75−81°, and 128−155°, respectively. For the graphene-coated THF monolayer, θ peaks up at about 42°, and for graphene-coated THF bilayer, θ exhibits three peaks at around 20°, 70°, and 110°, respectively. Compared to adlayers without the graphene coating, the θ peaks for the graphenecoated adlayers move to lower values. Nevertheless, the graphene coating still preserves orientations of THF molecules (Figure 4), and the graphene-coated adlayers still maintain certain liquid-like properties as the adlayers without the graphene coating. In Figure S2, Supporting Information, we also plot θ distribution for the THF molecules in the first and second adlayers of the bilayer, respectively, without and with the graphene coating. Both distributions exhibit similar three-peak features. Without the graphene coating, one difference in the θ distribution between the THF molecules in the first and second adlayers is the location of the first peak. For the second adlayer, the first peak is at ∼40°, whereas for the first adlayer, the first peak is in the range of 0−20° (see Figure S2a, Supporting Information). Hence, some THF molecules in the first adlayer tend to stand up. However, for the graphene-coated bilayer, a significant difference between the orientations of the THF molecules in the first and second adlayers is that one of the three main peaks of orientation-angle distribution for THF molecules in the first adlayer is located around 100°, whereas the corresponding peak for THF molecules in the second adlayer largely diminishes, and the peak around 24° becomes the highest one (see Figure S2b, Supporting Information). These θ preferences for graphene-coated THF bilayer suggest that more THF molecules in the second adlayer are in standing-up configurations than the first adlayer. The overall thickness of the bilayer (h2) is closer to the corresponding thickness of two crystalline layers (8.94 Å), compared to the 21897

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Figure 5. Atomic density profiles of (a) oxygen, (b) carbon, and (c) hydrogen atoms of THF molecules in monolayer and bilayer without and with graphene coating, respectively. The blue lines highlight values of layer thickness of monoclinic THF crystal along the b axis.14

Figure 6. Probability distribution of (a) intermolecular distance (r) and (b) rotation angle (ω) between two THF molecules in monolayer and bilayer without and with graphene coating, respectively. The intermolecular distance and rotation angle are defined as the distance of center of mass and angle of molecular axes between two THF molecules. The blue bars indicate intermolecular distances and rotation angles of crystal packing, respectively.14

evaporation of the THF molecules from the mica substrate, it still allows the THF molecules to freely flip up or down. In Figure 5, atomic density profiles of the THF molecules are plotted. By adding the graphene coating on the monolayer, the peak value of the O-profile [O atoms in the THF rings] (Figure 5a) approaches to the value of a crystalline layer, while the peaks of profiles of vertical height of C atoms and H atoms (Figures 5b,c) are shifted to lower values. For the bilayer, again the graphene coating leads to a sharper peak of the O-profile, whose value is close to that of two crystalline layers (Figure 5a). Intermolecular Translation and Rotation of THF Molecules. Consistent to the conclusion from the AFM experiment,7 with the graphene coating, the THF monolayer and the first adlayer are liquid-like but likely have a higher viscosity than that of bulk liquid. However, the graphene-coated second adlayer behaves more like a solid. In Figure 6, we plot the distribution of intermolecular distance r (r is defined as the distance between the centers of mass of two THF molecules) for the four systems. For the graphene-coated monolayer, values of two highest peaks are close to the possible intermolecular distances (∼5.4 and ∼9.8 Å) between two THF molecules in the crystal packing (see Figure 6a). In the perfect THF monoclinic crystal on mica, the THF molecules exhibit only two configurations, i.e., O atoms pointing away from or toward the substrate. The corresponding rotation angle (ω), defined as the angle between molecular axes of two THF molecules, can be either 0° or 180°. As shown in Figure 6b, the ω distribution spans over the entire scale (0−180°), suggesting the THF molecules can freely rotate

case of the monolayer (Figure 3), because the second adlayer is much less diffusive than the first adlayer. For the model of mica considered in this study, the mica surface is electron-rich due to the top O atoms. The polar THF molecule has a five-membered ring. Within the THF ring, the electron-rich site is located at the O atom. Because of the strong electrostatic interaction, the THF molecules in the first THF adlayer would favor the upward orientation so that the O atoms in the rings can be away from the O atoms on mica. In the initial configuration of each model shown in Figure 1, 60% of the THF molecules within the first adlayer are set in the downward orientation (see Table S1, Supporting Information). After 30 ps BOMD simulations, the statistically averaged downward configurations for the THF molecules in the first adlayer (of the bilayer) are reduced to ∼40% (Table S1, Supporting Information), and those for the graphene-coated monolayer are even reduced to ∼10%, suggesting that most downward orientation are switched to the upward orientation due to the molecule−mica interaction. Without the graphene coating, the orientation of THF molecules in the second adlayer exhibits little change from the initial configuration. However, with the graphene coating, most THF molecules in the second adlayer tend to adopt the upward orientation. The increased number of upward orientations for the THF molecules is possibly due to the favorable polarization interaction between the graphene and O atoms on the THF molecules. The change in the orientation of THF molecules indicates that although the graphene coating blocks the 21898

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Initiative, the UNL NCESR, and from USTC for (1000plan) Qianren-B summer research.

even with the graphene coating. Since most THF molecules in the graphene-coated monolayer and bilayer exhibit upward orientation, the related ω distribution is most likely populated in the angle range ω < 90°. In Figure S3, Supporting Information, we present rotation-angle distribution of THF molecules in the first and second adlayers of the bilayer, respectively, without and with graphene coating. Without graphene coating, the ω distribution of the second adlayer still spans over the whole scale, but the ω distribution of the first adlayer has a two-peak feature (peaks at 40−80° and 100− 140°). For graphene-coated bilayer, the ω distribution of the second adlayer is much narrower in range (40−90°) than that of the first adlayer (50−120°), suggesting more ordered packing for the second adlayer.

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(1) Geim, A. K.; Novoselov, K. S. The Rise of Graphene. Nat. Mater. 2007, 6, 183−191. (2) Geim, A. K. Graphene: Status and Prospects. Science 2009, 324, 1530−1534. (3) Nilsson, L.; Andersen, M.; Balog, R.; Lægsgaard, E.; Hofmann, P.; Besenbacher, F.; Hammer, B.; Stensgaard, I.; Hornekær, L. Graphene Coatings: Probing the Limits of the One Atom Thick Protection Layer. ACS Nano 2012, 6, 10258−10266. (4) Mu, R.; Fu, Q.; Jin, L.; Yu, L.; Fang, G.; Tan, D.; Bao, X. Visualizing Chemical Reactions Confined under Graphene. Angew. Chem., Int. Ed. 2012, 51, 4856−4859. (5) Yuk, J. M.; Park, J.; Ercius, P.; Kim, K.; Hellebusch, D. J.; Crommie, M. F.; Lee, J. Y.; Zettl, A.; Alivisatos, A. P. High-Resolution EM of Colloidal Nanocrystal Growth Using Graphene Liquid Cells. Science 2012, 336, 61−64. (6) Xu, K.; Cao, P.; Heath, J. R. Graphene Visualizes the First Water Adlayers on Mica at Ambient Conditions. Science 2010, 329, 1188− 1191. (7) Cao, P.; Xu, K.; Varghese, J. O.; Heath, J. R. Atomic Force Microscopy Characterization of Room-Temperature Adlayers of Small Organic Molecules through Graphene Templating. J. Am. Chem. Soc. 2011, 133, 2334−2337. (8) Cao, P.; Xu, K.; Varghese, J. O.; Heath, J. R. The Microscopic Structure of Adsorbe Water on Hydrophobic Surfaces under Ambient Conditions. Nano Lett. 2011, 11, 5581−5586. (9) Shim, J.; Lui, C. H.; Ko, T. Y.; Yu, Y.-J.; Kim, P.; Heinz, T. F.; Ryu, S. Water-Gated Charge Doping of Graphene Induced by Mica Substrates. Nano Lett. 2012, 12, 648−654. (10) Cao, P.; Varghese, J. O.; Xu, K.; Heath, J. R. Visualizing Local Doping Effects of Individual Water Clusters on Gold(111)-Supported Graphene. Nano Lett. 2012, 12, 1459−1463. (11) He, K. T.; Wood, J. D.; Doidge, G. P.; Pop, E.; Lyding, J. W. Scanning Tunneling Microscopy Study and Nanomanipulation of Graphene-Coated Water on Mica. Nano Lett. 2012, 12, 2665−2672. (12) Li, H.; Zeng, X. C. Two Dimensional Epitaxial Water Adlayer on Mica with Graphene Coating: An ab Initio Molecular Dynamics Study. J. Chem. Theory Comput. 2012, 8, 3034−3043. (13) Kuwahara, Y. Muscovite Surface Structure Imaged by Fluid Contact Mode AFM. Phys. Chem. Miner. 1999, 26, 198−205. (14) Luger, P.; Buschmann, J. Twist Conformation of Tetrahydrofuran in the Crystal. Angew. Chem., Int. Ed. 1983, 22, 410. (15) Lippert, G.; Hutter, J. R.; Parrinello, M. A Hybrid Gaussian and Plane Wave Density Functional Scheme. Mol. Phys. 1997, 92, 477− 487. (16) VandeVondele, J.; Krack, M.; Mohamed, F.; Parrinello, M.; Chassaing, T.; Hutter, J. QUICKSTEP: Fast and Accurate Density Functional Calculations using a Mixed Gaussian and Plane Waves Approach. Comput. Phys. Commun. 2005, 167, 103−128. (17) Becke, A. D. Density-Functional Exchange-Energy Approximation with Correct Asymptotic Behavior. Phys. Rev. A 1988, 38, 3098−3100. (18) Lee, C.; Yang, W.; Parr, R. G. Development of the Colle− Salvetti Correlation-Energy Formula into a Functional of the Electron Density. Phys. Rev. B 1988, 37, 785−789. (19) Goedecker, S.; Teter, M.; Hutter, J. Separable Dual-Space Gaussian Pseudopotentials. Phys. Rev. B 1996, 54, 1703−1710. (20) Hartwigsen, C.; Goedecker, S.; Hutter, J. Relativistic Separable Dual-Space Gaussian Pseudopotentials from H to Rn. Phys. Rev. B 1998, 58, 3641−3662. (21) VandeVondele, J.; Hutter, J. Gaussian Basis Sets for Accurate Calculations on Molecular Systems in Gas and Condensed Phases. J. Chem. Phys. 2007, 2007, 114105. (22) Grimme, S. Semiempirical GGA-Type Density Functional Constructed with a Long-Range Dispersion Correction. J. Comput. Chem. 2006, 27, 1787−1799.

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CONCLUSIONS The first-principles molecular dynamics simulations are performed to further illustrate the influence of the graphene coating on dynamic and structural properties of the THF molecules on mica at room temperature. On the basis of the computed self-diffusion coefficients of the THF molecules in different systems, we find that without the graphene coating, the motion of THF molecules on mica is the same as pure THF solvent. However, the graphene-coated monolayer and the first adlayer of graphene-coated bilayer become more viscous with slower motion; and the second adlayer of the graphene-coated bilayer even exhibits solid-like behavior. Although the motion of THF molecules is limited by the graphene coating, these molecules can freely tilt and rotate on mica surface. Therefore, the graphene-coated THF monolayer, bilayer, and even the solid-like second adlayer of the bilayer are not as ordered as the monoclinic THF crystal. However, in graphene-coated monolayer and bilayer, some THF molecules still keep crystal packing. These liquid−solid-like THF adlayers form a new phase, ranged between regular liquid and solid states, which is in good agreement with the AFM observation.7 Besides the icelike water adlayers on mica due to the graphene coating,6 our BOMD simulations show that such a liquid−solid state can also exist in non-hydrogen-bonded systems, implying that graphene coating can be used to image highly volatile molecules by constraining their motion and achieve supercooled liquid at room temperature.

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ASSOCIATED CONTENT

S Supporting Information *

Movies of THF adlayers on mica without and with graphene coating, variations of MSD of THF molecules with time, and statistical results about orientation and rotation of the THF molecules are included. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*(X.C.Z.) E-mail: [email protected]. Tel: 402-472-9894. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS P.C. was supported by the Department of Energy, Basic Energy Sciences (DE-FG03-01ER46175) (James Heath PI). X.C.Z. is supported by grants from the NSF (CBET-1066947 and CHE1306326), ARL (W911NF1020099), the Nebraska Research 21899

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The Journal of Physical Chemistry C

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(23) Holz, M.; Mao, X.-A.; Seiferling, D.; Sacco, A. Experimental Study of Dynamic Isotope Effects in Molecular Liquids: Detection of Translation-Rotation Coupling. J. Chem. Phys. 1996, 104, 669−679.

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dx.doi.org/10.1021/jp408589j | J. Phys. Chem. C 2013, 117, 21894−21900