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Chemical Understanding of the Mechanisms Involved in Mitigation of Charged Impurity Effects by Polar Molecules on Graphene Barrett Cameron Worley, Ryan T. Haws, Peter J Rossky, and Ananth Dodabalapur J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b03853 • Publication Date (Web): 26 May 2016 Downloaded from http://pubs.acs.org on June 5, 2016

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The Journal of Physical Chemistry

Chemical Understanding of the Mechanisms Involved in Mitigation of Charged Impurity Effects by Polar Molecules on Graphene Barrett C. Worley1,2, Ryan T. Haws4, Peter J. Rossky4 *, and Ananth Dodabalapur1,3* 1

Microelectronics Research Center, The University of Texas at Austin, Austin, TX 78758, USA

2

Department of Chemistry, The University of Texas at Austin, Austin, TX, 78712, USA

3

Department of Electrical and Computer Engineering, The University of Texas at Austin,

Austin, TX 78712, USA 4

Department of Chemistry, Rice University, Houston, TX, 77005-1892, USA

Abstract

It is well known that the transport properties of monolayer graphene are degraded by charged impurities present between graphene and either a given substrate or air. Such impurities cause charge scattering of holes and electrons in graphene. In previous work, our group has used both fluoropolymer thin films and polar vapor molecules to dramatically improve graphene fieldeffect transistor (FET) device characteristics, including Dirac voltage and mobility. We attributed the graphene device improvements to mitigation of charged impurities and defects due

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to electrostatic interaction with the dipoles of the applied fluoropolymers and polar molecules. In this work, we present theoretical support to this hypothesis, in the form of computational chemical simulations involving the interaction of polar molecules and impurities on a graphene sheet. We examine two types of impurities which may occur at graphene interfaces: ionic impurities and molecular dipole impurities. Upon introduction of polar vapor molecules to an impurity/graphene system, we observed a dramatic reduction in the electrostatic potential in the plane of the graphene from the impurity. The magnitude of potential reduction scales with the average dipole moment of each polar molecule. We were able to determine two separate mechanisms which contribute to the total potential reduction, impurity displacement and electrostatic screening of the impurity. The respective impacts of the mechanisms vary with distance from the impurity. Additionally, in the case of the molecular dipole impurity, the orientation of the impurity atop graphene is a key factor that determines the potential impact.

Introduction Graphene, graphene nanoribbons, and other emerging 2-D materials show great promise for use in a variety of microelectronics applications

1-12

. However, due to scattering effects from

impurities and defects which incorporate in and around the materials during fabrication processes, graphene and similar materials exhibit transport metrics well below theoretical limits 6-7, 13-29

. Specifically, long-range, Coulombic scattering is the dominant scattering mechanism in

the graphene layer of microelectronic devices fabricated via the popular CVD graphene growth and wet-transfer to substrate method

1, 4, 15, 18, 21, 24, 29-36

. Measured graphene device properties

such as charge carrier mobilities, conductivity, and on/off ratios are much lower than predicted in theoretical calculations. Thus, it is imperative that we find ways to mitigate the effects of charged impurities and defects on graphene and similar 2-D materials.


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Our previous work has shown that fluoropolymer capping layers provide dramatic improvements to the electrical characteristics of graphene devices

23-25

. We found that the

fluoropolymers returned the Dirac voltage toward zero gate voltage, whether the Dirac peak was positively or negatively shifted. In many cases, the magnitude of the shift was large (>50 V). At the same time, both electron and hole mobility values increase and residual carrier concentration decreased. We hypothesized that these improvements were due both to the polar fluoropolymers’ screening of charged impurities and defects at the graphene monolayer and to a change in the dielectric environment around graphene. We followed these experiments with model experimental studies using polar, vapor-phase molecules delivered to a graphene device surface 22

. These molecules also acted to mitigate the effects of charged impurities present at the

graphene/substrate or graphene/air interface(s), thus improving key electrical characteristics of the devices. We attribute this mitigation to electrostatic interaction between the dipoles of the polar vapor molecules and the charged impurities. The reversible nature of the improvements to the device upon removal of the fluoropolymers or dissipation of the polar molecules supported our hypothesis that the effect was caused by electrostatics, as opposed to covalent bond formation or some other electronic interaction. Hiroki Ago and coworkers reported similar conclusions regarding the nature of the interaction between piperidine molecules and graphene 37

. See Supporting Information Figure 1 for our vapor-phase results. In the current work, we studied the mechanism behind polar molecules’ mitigation of charged

impurities at a graphene surface by performing computational chemical simulations consisting of polar molecules interacting with impurities on a graphene sheet in vacuum. We performed molecular dynamics (MD) studies of a graphene sheet in a periodic unit cell, approximating an infinite graphene sheet. To represent both the charged ion and molecular dipole classes of


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impurity, we added a sodium cation and a single water molecule, respectively, above the graphene. These two impurities are ubiquitous in microelectronic fabrication, and there is much literature detailing attempts to minimize their effects on devices

36, 38-41

. NVT simulations for

each graphene/impurity system both with and without polar molecules gave an ensemble of structures from which to calculate the impurity’s electrostatic potential profile in the plane of the graphene sheet. Figure 1 shows an example image of an acetone/sodium/graphene system. We then calculated the changes in potential between each system with and without polar molecules to study the mechanisms governing how polar molecules change the effects of impurities on graphene. Methods We used the Carbon Nanostructure Builder (version 1.2) extension in VMD software (version 1.9.1) to construct a 5 nm by 5 nm graphene sheet consisting of 1008 carbon atoms. We carried out simulations using version 4.5.3 of the GROMACS molecular dynamics software package with the OPLS-AA force field, starting with steep energy minimization of the graphene sheet to a maximum force of 10 kJ/mol*nm

42-45

. Next, we ran a NVT (constant number of particles,

volume, and temperature) statistical ensemble simulation for 1 nanosecond at 300 K to equilibrate the sheet, followed by a production NVT simulation run for 10 nanoseconds at 300 K. In the final step of preparing the graphene sheet, we ran a NPT (constant number of particles, pressure, and temperature) simulation to optimize the size of the unit cell box around the sheet. With a stable system of a graphene sheet inside a periodic unit cell, we then added, separately, a sodium ion of charge positive 1e and a water molecule about 3.5 Angstroms above the sheet. We used the three-site SPC/E water model. While there is ongoing debate in the literature regarding the effects of different water models, polarizability, and other factors on the interaction of liquid


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water with a hydrophobic surface, such as graphene, we have not included such polarization effects in our calculations. We believe that such issues would be appropriate in a more detailed study specific to water/graphene interactions. Next, we treated each impurity/graphene system to successive steps of energy minimization down to a maximum residual force of 0.5 kJ mol/mol*nm, NVT equilibration, and 10 nanosecond NVT production simulations. After collecting respective ensembles for each type of impurity, we added 50 molecules of each type of polar vapor. We chose to model the vapors used in our previous experimental work, specifically, acetone, ethanol, and IPA. We then collected respective ensembles for each polar vapor/impurity/graphene system by treating them to the same successive simulations as were done to the impurity/graphene systems. It is evident from viewing the simulation trajectories in VMD that, over the course of the NVT runs, the polar molecules both solvate each impurity and orient their dipoles so as to screen the electrostatic charge of the sodium ion or dipole moment of the water impurity, as expected. After collecting ensembles from simulations, we calculated the average potentials from each impurity in the plane of the graphene, both with and without polar vapor molecules present. The code first evaluates the atomic coordinates in each simulation time step, and then calculates the radial potential out to 1 nm from a point on the graphene plane directly beneath the centroid of the impurity atom(s). We can then extract data on the separation distance of the impurity from the graphene sheet, the electrostatic potential in the plane of the graphene sheet from each impurity alone, the potential after inclusion of polar molecules, and the potential when only the displacement of the impurity away from the sheet by polar molecules is considered. Further, we can calculate both the total magnitude of potential reduction by the polar molecules and the potential only from electrostatic screening of the impurity by polar molecules. All potentials are


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plotted as a function of distance on the sheet from the impurity. Next, we can measure the angles of the water molecule bisector from the z-axis, to observe the orientation(s) of the water impurity over the ensemble, both with and without the presence of polar molecules. The cosines of the measured angles range from -1 to 1, where -1 represents both hydrogens of the water molecule pointing down toward the graphene sheet, 0 represents the water parallel to the sheet, and 1 represents both hydrogens pointing up, away from the sheet. We plotted the probability distributions for these angle data for both the water impurity alone on graphene and with various polar molecules. Finally, we can calculate the range of dipole moments of the various polar molecules, using the charges and positions of their respective atoms averaged over each ensemble of our simulations, and plot these as probability distributions as well. Results and Discussion A plot of the average radial electrostatic potential in the plane of the graphene sheet from a sodium ion impurity is shown in Figure 2a. The potential is plotted in two dimensions from the point on the sheet directly below the impurity, and extends radially to a distance of one nanometer (nm). Figure 2b shows a “slice” of the radial potential plot, with the potential as a function of distance in one direction. The potential in the plane of the graphene directly under the sodium ion in the absence of polar molecules is large, and rapidly decays over distance away from the ion. However, inclusion of polar molecules, such as acetone, solvates the sodium ion and significantly reduces the potential at the graphene sheet. Figure 2c shows the impurity potential on graphene as a function of distance after inclusion of 50 of each type of polar molecule in separate simulations. Acetone (ACE), ethanol (ETOH), and isopropyl alcohol (IPA) were chosen because they were also used in our experimental work. The observation that polar molecules with greater dipole moments cause greater reduction of calculated impurity potentials


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(Figure 2c and Supporting Information Figure 2) correlates well with the similar trend for device characteristic improvements observed in previous work 22. Taking the acetone/sodium ion/graphene case as a general example of the interaction of polar molecules with impurities on graphene, we first calculated the difference in potential between the system with and without acetone molecules. The magnitude by which the acetone molecules reduced the potential is shown as the solid line in Figure 3a. We identified two mechanisms by which the polar acetone molecules act to mitigate the potential profile in the plane of the graphene sheet from the sodium ion, as indicated in Figure 3a. The first mechanism involves physical displacement of the sodium ion away from the sheet upon solvation by acetone molecules (Supporting Information Figure 3).

The second mechanism is the electrostatic

screening of the sodium ion’s charge by orientation of the acetone dipoles around the cation. These two are distinct effects, although not completely separable, since the screening would be quantitatively affected by the ion position. The average contribution of each of these mechanisms to the potential reduction varies with distance and is shown as a percentage in Figure 3b. While displacement is the dominant mechanism at short range from the impurity, screening is increasingly the primary contributor to potential reduction at distances beyond about 0.26 nm, as expected since the ion displacement from the surface is small. Additionally, we studied the effects of varying the number of polar molecules per impurity on potential reduction by repeating the 50-molecule acetone simulation with 10 and 25 molecules per unit cell (Supporting Information Figure 4). We observed that greater numbers of polar molecules such as acetone provide both greater magnitudes of potential reduction (Figure 4a), and greater displacement of the sodium impurity (Figure 4b). Figure 4c lends insight into the dependence on the number of molecules for formation of solvation shells around the sodium ion.


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For only ten acetone molecules present, the distribution of distances between sodium ion and graphene plane is roughly Gaussian with a center at ~3.5 Angstroms, representing a single solvation shell of acetone molecules around the sodium ion. This distribution becomes bimodal as more acetone molecules are introduced to the simulation box, representing the formation of a second shell. Other polar molecules, including a variety of alcohols, simulated with sodium gave results similar to those shown for acetone (Supporting Information Figure 5). One important conclusion from this analysis is that the great majority of the screening can be achieved with adsorption of only a few polar molecules. Water, either adsorbed on top of graphene or trapped between graphene and substrate, may be observed to affect graphene’s electrical properties in shifts of the Dirac voltage peak or charge neutral point and hysteresis in graphene FET transfer curves

37, 41, 46-48

. There is much literature

covering extensive calculations of the orientation-dependent water-graphene interaction. For our simulations, we chose two orientations of greatest interaction energy: the “one-leg” orientation, with one hydrogen-oxygen bond vector pointing down toward the graphene; and the “two-leg” orientation, where both hydrogens point toward graphene

49-51

. As shown in Figure 5a and 5b,

these two orientations of water dipoles cause relatively similar radial potentials in the plane of the graphene in our simulations. The greatest difference in potential between the one-leg orientation and the two-leg orientation is about 0.075 e/nm at 0.2 nm distance on the graphene plane from the point directly below the water molecule (Figure 5c). We did not observe significant differences between our results for each orientation, so, for the remainder of this discussion, we will present data in relation to the one-leg water orientation; the data for the twoleg orientation case is provided in the Supporting Information section.


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When polar molecules are added, they act to solvate the water impurity, as shown with acetone molecules in Figure 6a. For acetone and other polar molecules, this results in a reduction of the potential magnitude in the graphene plane caused by water (Figure 6b). Figure 6c reveals that the polar molecules actually change the potential in the graphene plane induced by the water from positive to negative potential. Nevertheless, the magnitude of potential reduction by polar molecules does correlate with their respective dipole moments (Figure 6c and Supporting Information Figure 6), and thus supports both our previous experimental data and hypothesis that it is the polar nature of these molecules which acts to mitigate charge scattering effects of impurities on graphene via a reduction of their electrostatic potential 22. However, the reversal in potential sign from positive to negative indicates a more complex process, which we will discuss later. As was the case with polar molecules and a sodium ion impurity on graphene, we observed two mechanisms apparently governing the polar molecules’ reduction in water impurity electrostatic potential in the plane of the graphene sheet. For acetone molecules, the magnitude of potential reduction, along with the contribution of both water displacement and electrostatic screening mechanisms, is shown in Figure 7a. The percentage contribution that each mechanism makes to the total potential reduction is shown as a function of distance in Figure 7b. Displacement of the water impurity appears to be a significant mechanism for potential reduction at short range, and screening becomes the dominant mechanism at distance beyond about 0.26 nm. However, we note that, as compared to displacement and screening mechanism percentage contributions for potential reduction in the case of acetone molecules and a sodium ion impurity, the percentage contributions of the mechanisms for potential reduction in the case of acetone molecules and a water impurity are significantly more balanced at close range. So, while


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screening remains the dominant mechanism at long range, displacement of a water impurity is not as dominant at short range. To investigate why displacement of a water impurity appears to have reduced impact as compared with displacement of a sodium ion by the same acetone molecules, we varied the number of acetone molecules used in the simulation. We repeated the 50-molecule acetone simulation with numbers of 10 and 25 molecules per unit cell. As shown in Figure 8a, greater numbers of acetone molecules provide greater magnitudes of potential reduction. However, there is not an apparent trend in effects of the number of acetone molecules on displacement of water (Figure 8b), and acetone molecules do not appear to form distinct, successive solvation shells beyond the first one (Figure 8c). This difference in displacement trends for acetone with a water impurity versus with a sodium ion impurity further hints at greater complexity in the mechanisms behind water impurity potential reduction. For similar data with ethanol molecules, see Supporting Information Figure 7. For data with the two-leg water orientation and various additional polar molecules, see Supporting Information Figure 8. When considering both the change in sign of the potential and limited displacement of the water impurity upon introduction of polar molecules, the strict, two-mechanism hypothesis is not sufficient to explain the impurity potential change. Hence, it is instructive to consider the dipolar nature of the water molecule (and, by extension, the molecular dipole impurity class), and observe how the orientation of the water molecule changes in the presence of polar molecules. Thus far, our data considered the water molecule to be locked into the preferred one-leg orientation atop graphene, as it is in the solvent-free reference geometry. However, Figure 9a, a plot of the cosine of the angle between the water molecule bisector, or dipole vector, and the zaxis above the x-y graphene plane, illustrates how acetone molecules change the preferred


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orientation of the water molecule atop graphene. Where the angle cosine is -1, the water hydrogens are pointed down toward the graphene. At angle cosine 0, the water has hydrogens and oxygen atom parallel to the graphene sheet, and, at angle cosine +1, the hydrogens are pointing away from the sheet. We note that the simulation ensemble shows that the acetone molecules orient the water such that it is most probable to have the oxygen pointing more toward (and hydrogens pointing away from) the graphene sheet. This orientation is more likely to cause a negative potential in the plane of the graphene sheet, as is seen in Figure 6c. Additionally, ethanol (Figure 9b) and IPA Figure 9c) molecules tend to orient the water molecule parallel to the sheet, with some skew toward oxygen down, and this helps explain the less negative potential observed for alcohols versus acetone plots in Figure 6c. This theoretical work strongly supports our previous experimental efforts to show that it is not difficult to both access the impurities/defects in 2-D monolayer graphene and manipulate them in order to improve transport characteristics. Access to these 2-D material impurities/defects presents a distinct advantage for such materials over comparable bulk conductive materials. While manipulation of impurities/defects with polar molecules is a good method for graphene, there may be other modification methods more appropriate for other 2-D materials such as MoS2. Conclusions In summary, if a sodium ion near graphene is thought of as a point charge impurity or a water molecule near graphene is considered as representative of any type of molecular dipole impurity which scatters charge carriers moving through the monolayer, then the magnitude of that scattering should scale with the magnitude of the electrostatic potential in the graphene plane from that impurity. As we have demonstrated theoretically, polar molecules reduce that electrostatic potential via simple mechanisms. In the case of a point charge, physical


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displacement and electrostatic screening are sufficient mechanisms to greatly reduce the potential. While displacement is more important for potential reduction at short range, screening dominates at long range from the impurity. For a molecular dipole, displacement, screening, and reorientation of the impurity are responsible for potential reduction. Despite the added complexity of the displacement and reorientation mechanisms, electrostatic screening of the water impurity by polar molecules does dominate the potential reduction at longer range from the impurity. This reduction in impurity potential on graphene strongly supports our previous experimental observations that polar molecules mitigate the effects of charged impurities on graphene, which was observed as improvement in graphene device characteristics.

Figure 1. A system containing 50 acetone molecules, a sodium ion impurity, and a graphene sheet.


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Figure 2. a) A sodium ion atop graphene, with radial electrostatic potential distribution shown by colored circles. b) Electrostatic potential in the plane of the graphene sheet from the sodium ion as a function of distance. c) Potential plots after inclusion of polar molecules in the simulation, showing dramatic reduction of the potential at graphene by polar molecules.

Figure 3. a) Potential plot showing the magnitude of potential reduction by acetone molecules (solid line), contribution of displacement mechanism to this potential reduction (dashed line), and contribution of screening mechanism (dot-dot-dashed line). The inset shows the two potential plots of sodium on graphene with (red) and without (black) acetone molecules. b) Percentage contribution of each mechanism to the potential reduction.


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Figure 4. a) Potential plots for sodium both alone and with various numbers of acetone molecules. b) Box plots showing mean displacement (small square) of sodium ion away from graphene as a function of number of acetone molecules, with standard error (larger box boundaries). c) Distributions of values of sodium ion displacement by acetones, revealing formation of a second solvation shell with greater numbers of acetone molecules.


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Figure 5. Schematics showing potential profile in the plane of the graphene sheet from one water molecule with a) one-leg and b) two-leg water orientations. c) Electrostatic potential as a function of distance on graphene from one-leg and two-leg water orientations.

Figure 6. a) A system containing 50 acetone molecules, a water molecule impurity, and a graphene sheet. b) Absolute value of potential from one-leg water orientation both alone and with various polar molecules. c) Change in potential caused by polar molecules for water.

Figure 7. a) Potential plot showing the magnitude of potential reduction for an adsorbed water molecule impurity by acetone molecules (solid line), contribution of displacement mechanism to this potential reduction (dashed line), and contribution of screening mechanism (dot-dot-dashed line). b) Percentage contribution of each mechanism to the potential reduction.


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Figure 8. a) Potential plots for water alone and with various numbers of acetone molecules. b) Box plots showing mean displacement (small square) of water away from graphene as a function number of acetone molecules, with standard error (larger box boundaries). c) Distributions of values of water displacement by acetone molecules, indicating formation of only one solvation shell around water.


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Figure 9. Distributions of values of the angle between water dipole vector and z-axis above graphene sheet in the presence of a) acetone b) ethanol c) IPA molecules. Angle cosine -1: water hydrogens are pointed down toward the graphene. Angle cosine 0: the water hydrogens and oxygen atoms parallel to graphene. Angle cosine +1: hydrogens are pointing away from the graphene. ASSOCIATED CONTENT Supporting Information. Polar vapor results for previous experimental work, probability distributions for dipole moments of various polar molecules calculated from their respective atoms’ positions and point charges in graphene/sodium ion simulations, comparison of sodium ion position relative to graphene before and after solvation by polar molecules, comparison of varying numbers of acetone molecules per graphene/sodium simulation system, electrostatic potential as a function of distance and sodium ion distance from graphene data for various alcohols, probability distributions for dipole moments of various polar molecules calculated from their respective atoms’ positions and point charges in graphene/water simulations, data for various numbers of ethanol molecules of water impurity electrostatic potential as a function of distance and water molecule distance from graphene, two-leg water orientation electrostatic potential as a function of distance data for various polar molecules, and acetone molecule’s mechanistic effects on the two-leg water orientation electrostatic potential as a function of distance. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]


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*E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We gratefully acknowledge support from NSF ECCS Division, NSF Cooperative Agreement No. EEC-1160494 (Nascent). PJR acknowledges the support of the National Science Foundation (CHE-1362381). We also thank Prof. Deji Akinwande for helpful discussions. REFERENCES 1. Adam, S.; Hwang, E. H.; Galitski, V. M.; Sarma, S. D., A Self-Consistent Theory for Graphene Transport. PNAS 2007, 104, 18392-18397. 2. Avouris, P., Graphene: Electronic and Photonic Properties and Devices. Nano Letters 2010, 10, 4285-4294. 3. Bolotin, K. I.; Sikes, K. J.; Hone, J.; Stormer, H. L.; Kim, P., Temperature-Dependent Transport in Suspended Graphene. Phys. Rev. Lett. 2008, 101, 096802-1 - 096802-4. 4. Bolotin, K. I.; Sikes, K. J.; Jiang, Z.; Klima, M.; Fudenberg, G.; Hone, J.; Kim, P.; Stormer, H. L., Ultrahigh Electron Mobility in Suspended Graphene. Solid State Communications 2008, 146, 351-355. 5. Castro Neto, A. H.; Guinea, F.; Peres, N. M. R.; Novoselov, K. S.; Geim, A. K., The Electronic Properties of Graphene. Rev. Mod. Phys. 2009, 81, 109-162. 6. Chan, J.; Venugopal, A.; Pirkle, A.; McDonnell, S.; Hinojos, D.; Magnuson, C. W.; Ruoff, R. S.; Colombo, L.; Wallace, R. M.; Vogel, E. M., Reducing Extrinsic PerformanceLimiting Factors in Graphene Grown by Chemical Vapor Deposition. ACS Nano 2012, 6, 32243229. 7. Chen, J. H.; Jang, C.; Ishigami, M.; Xiao, S.; Cullen, W. G.; Williams, E. D.; Fuhrer, M. S., Diffusive Charge Transport in Graphene on SiO2. Solid State Communications 2009, 149, 1080-1086. 8. Das Sarma, S.; Adam, S.; Hwang, E. H.; Rossi, E., Electronic Transport in TwoDimensional Graphene. Rev. Mod. Phys. 2011, 83, 407-470.


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ToC Figure. A sodium ion atop graphene, with radial electrostatic potential distribution mitigated by surrounding acetone molecules.


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Figure 1. A system containing 50 acetone molecules, a sodium ion impurity, and a graphene sheet. 171x136mm (106 x 106 DPI)



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Figure 2. a) A sodium ion atop graphene, with radial electrostatic potential distribution shown by colored circles. b) Electrostatic potential in the plane of the graphene sheet from the sodium ion as a function of distance. c) Potential plots after inclusion of polar molecules in the simulation, showing dramatic reduction of the potential at graphene by polar molecules. 256x67mm (150 x 150 DPI)


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Figure 3. a) Potential plot showing the magnitude of potential reduction by acetone molecules (solid line), contribution of displacement mechanism to this potential reduction (dashed line), and contribution of screening mechanism (dot-dot-dashed line). The inset shows the two potential plots of sodium on graphene with (red) and without (black) acetone molecules. b) Percentage contribution of each mechanism to the potential reduction. 230x85mm (150 x 150 DPI)



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Figure 4. a) Potential plots for sodium both alone and with various numbers of acetone molecules. b) Box plots showing mean displacement (small square) of sodium ion away from graphene as a function of number of acetone molecules, with standard error (larger box boundaries). c) Distributions of values of sodium ion displacement by acetones, revealing formation of a second solvation shell with greater numbers of acetone molecules. 184x131mm (150 x 150 DPI)


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Figure 5. Schematics showing potential profile in the plane of the graphene sheet from one water molecule with a) one-leg and b) two-leg water orientations. c) Electrostatic potential as a function of distance on graphene from one-leg and two-leg water orientations. 255x68mm (150 x 150 DPI)



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Figure 6. a) A system containing 50 acetone molecules, a water molecule impurity, and a graphene sheet. b) Absolute value of potential from one-leg water orientation both alone and with various polar molecules. c) Change in potential caused by polar molecules for water. 254x65mm (150 x 150 DPI)


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Figure 7. a) Potential plot showing the magnitude of potential reduction for an adsorbed water molecule impurity by acetone molecules (solid line), contribution of displacement mechanism to this potential reduction (dashed line), and contribution of screening mechanism (dot-dot-dashed line). b) Percentage contribution of each mechanism to the potential reduction. 230x84mm (150 x 150 DPI)



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Figure 8. a) Potential plots for water alone and with various numbers of acetone molecules. b) Box plots showing mean displacement (small square) of water away from graphene as a function number of acetone molecules, with standard error (larger box boundaries). c) Distributions of values of water displacement by acetone molecules, indicating formation of only one solvation shell around water. 178x125mm (150 x 150 DPI)


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Figure 9. Distributions of values of the angle between water dipole vector and z-axis above graphene sheet in the presence of a) acetone b) ethanol c) IPA molecules. Angle cosine -1: water hydrogens are pointed down toward the graphene. Angle cosine 0: the water hydrogens and oxygen atoms parallel to graphene. Angle cosine +1: hydrogens are pointing away from the graphene. 255x64mm (150 x 150 DPI)


Chemical Understanding of the Mechanisms ... - ACS Publications

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