Jan-Michael Y. Carrillo

Conductive Thin Films of Pristine Graphene by Solvent Interface Trapping

Graphites insolubility in conventional solvents is a major obstacle to its utilization. This challenge is typically addressed by chemical modification such as oxidation, followed by reduction. However, pristine graphene possesses superior properties as oxidation and reduction lead to degradation of the graphene. Here we demonstrate the use of an interfacial trapping technique to assemble laterally macroscopic films of pristine graphene that are up to 95% transparent. This is accomplished by modest sonication of natural flake graphite in a water/heptane mixture to form continuous films at the interface between two immiscible liquids. Furthermore, the graphene sheets readily climb hydrophilic solid substrates, forming a homogeneous thin film one to four layers thick. These films are composed of a network of overlapping graphene sheets and shown to have long-range structure with conductivities on the order of 400 S/cm.

Stabilization of graphene sheets by a structured benzene/hexafluorobenzene mixed solvent.

Applications requiring pristine graphene derived from graphite demand a solution stabilization method that utilizes an easily removable media. Using a combination of molecular dynamics simulations and experimental techniques, we investigate the solublization/suspension of pristine graphene sheets by an equimolar mixture of benzene and hexafluorobenzene (C(6)H(6)/C(6)F(6)) that is known to form an ordered structure solidifying at 23.7 °C. Our simulations show that the graphene surface templates the self-assembly of the mixture into periodic layers extending up to 30 Å from both sides of the graphene sheet. The solvent structuring is driven by quadrupolar interactions and consists of stacks of alternating C(6)H(6)/C(6)F(6) molecules rising from the surface of the graphene. These stacks result in density oscillations with a period of about 3.4 Å. The high affinity of the 1:1 C(6)H(6)/C(6)F(6) mixture with graphene is consistent with observed hysteresis in Wilhelmy plate measurements using highly ordered pyrolytic graphite (HOPG). AFM, SEM, and TEM techniques verify the state of the suspended material after sonication. As an example of the utility of this mixture, graphene suspensions are freeze-dried at room temperature to produce a sponge-like morphology that reflects the structure of the graphene sheets in solution.

Adhesion of nanoparticles.

We have developed a new model of nanoparticle adhesion which explicitly takes into account the change in the nanoparticle surface energy. Using combination of the molecular dynamics simulations and theoretical calculations we have showed that the deformation of the adsorbed nanoparticles is a function of the dimensionless parameter beta proportional, variant gamma(p)(GR(p))(-2/3)W(-1/3), where G is the particle shear modulus, R(p) is the initial particle radius, gamma(p) is the polymer interfacial energy, and W is the particle work of adhesion. In the case of small values of the parameter beta < 0.1, which is usually the case for strongly cross-linked large nanoparticles, the particle deformation can be described in the framework of the classical Johnson, Kendall, and Roberts (JKR) theory. However, we observed a significant deviation from the classical JKR theory in the case of the weakly cross-linked nanoparticles that experience large shape deformations upon particle adhesion. In this case the interfacial energy of the nanoparticle plays an important role controlling nanoparticle deformation. Our model of the nanoparticle adhesion is in a very good agreement with the simulation results and provides a new universal scaling relationship for nanoparticle deformation as a function of the system parameters.

Morphologies of Planar Polyelectrolyte Brushes in a Poor Solvent: Molecular Dynamics Simulations and Scaling Analysis

Using molecular dynamics simulations and scaling analysis, we study the effect of the solvent quality for the polymer backbone, the strength of the electrostatic interactions, the chain degree of polymerization, and the brush grafting density on conformations of the planar polyelectrolyte brushes in salt-free solutions. Polyelectrolyte brush forms: (1) vertically oriented cylindrical aggregates (bundles of chains), (2) maze-like aggregate structures, or (3) thin polymeric layer covering a substrate. These different brush morphologies appear as a result of the fine interplay between electrostatic and short-range monomer-monomer interactions. The brush thickness shows nonmonotonic dependence on the value of the Bjerrum length. It first increases with the increasing value of the Bjerrum length, and then it begins to decrease. This behavior is a result of counterion condensation within a brush volume.

New insights into the dynamics and morphology of P3HT:PCBM active layers in bulk heterojunctions

Organic photovoltaics (OPVs) are a topic of extensive research because of their potential application in solar cells. Recent work has led to the development of a coarse-grained model for studying poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) blends using molecular simulations. Here we provide further validation of the force field and use it to study the thermal annealing process of P3HT:PCBM blends. A key finding of our study is that, in contrast to a previous report, the annealing process does not converge at the short time scales reported. Rather, we find that the self-assembly of the blends is characterized by three rate dependent stages that require much longer simulations to approach convergence. Using state-of-the-art high performance computing, we are able to study annealing at length and time scales commensurate with devices used in experiments. Our simulations show different phase segregated morphologies dependent on the P3HT chain length and PCBM volume fraction in the blend. For short chain lengths, we observed a smectic morphology containing alternate P3HT and PCBM domains. In contrast, a phase segregated morphology containing domains of P3HT and PCBM distributed randomly in space is found for longer chain lengths. Theoretical arguments justifying stabilization of these morphologies due to shape anisotropy of P3HT (rod-like) and PCBM (sphere-like) are presented. Furthermore, results on the structure factor, miscibility of P3HT and PCBM, domain spacing and kinetics of phase segregation in the blends are presented in detail. Qualitative comparison of these results with published small-angle neutron scattering experiments in the literature is presented and an excellent agreement is found.

Detailed Molecular Dynamics Simulations of a Model NaPSS in Water

Hydrophobic polyelectrolytes are known to form necklace-like structures of dense beads connected by strings of monomers. This structure appears as a result of optimization of the electrostatic and short-range interactions. To elucidate the effect of counterion condensation and solvent on polyelectrolyte conformations, we performed two sets of molecular dynamics simulations of model poly(styrene)-co-styrene sodium sulfonate (NaPSS) chains with the degree of polymerization N = 16-64 and fraction of charged monomers f = 0.25-1 in aqueous solutions: (1) water molecules were considered explicitly using the TIP3P-PME model and (2) water molecules were modeled as a dielectric continuum with the dielectric constant 77.73. Our simulations showed that with increasing fraction of sulfonated groups f a polystyrene sulfonate chain adopts an elongated conformation. There is a transition between collapsed and elongated states which is manifested in the change of the scaling dependence of the chain size on the degree of polymerization. The effect of the water-ion interactions on counterion condensation was analyzed by comparing the radial distribution functions between the sulfonated groups and counterions for chains with different f values. In the case of the collapsed NaPSS chains, it was found that ionized groups are located at the globular surface.

Friction between brush layers of charged and neutral bottle-brush macromolecules. molecular dynamics simulations.

Using molecular dynamics simulations, we study the lubricating properties of neutral and charged bottle-brush coatings as a function of the compression and shear stresses and brush grafting density. Our simulations have shown that in charged bottle-brush systems under shear there is a layer with excess counterions located in the middle between brush-bearing surfaces. The main deformation mode of the charged bottle-brush layers is associated with the backbone deformation, resulting in the backbone deformation ratio, α, and shear viscosity, η, being universal functions of the Weissenberg number. In the case of neutral bottle-brush systems, in addition to the backbone deformation there is also side chain deformation. The coupling between backbone and side chain deformation violates universality in the deformation ratio, α, dependence on the Weissenberg number and results in scaling exponents varying with the compression stress and brush grafting density. The existence of different length scales controlling deformation of neutral bottle brushes manifests itself in the shear viscosity, η, dependence on the shear rate, ̇γ. Shear viscosity, η, as a function of the shear rate, ̇γ, has two plateaus and two shear thinning regimes. The low shear rate plateau and shear thinning regime correspond to the backbone deformation, while the second plateau and shear thinning regime at moderate shear rates are due to side chain deformation. For both systems the value of the friction coefficient increases with increasing shear rate. The values of the friction coefficient for charged bottle-brush systems are about ten times smaller than corresponding values for neutral systems at the same shear rate.

Contact mechanics of nanoparticles.

We perform molecular dynamics simulations on the detachment of nanoparticles from a substrate. The critical detachment force, f*, is obtained as a function of the nanoparticle radius, R(p), shear modulus, G, surface energy, γ(p), and work of adhesion, W. The magnitude of the detachment force is shown to increase from πWR(p) to 2.2πWR(p) with increasing nanoparticle shear modulus and nanoparticle size. This variation of the detachment force is a manifestation of neck formation upon nanoparticle detachment. Using scaling analysis, we show that the magnitude of the detachment force is controlled by the balance of the nanoparticle elastic energy, neck surface energy, and energy of nanoparticle adhesion to a substrate. It is a function of the dimensionless parameter δ ∝ γ(p)(GR(p))(-1/3)W(-2/3), which is proportional to the ratio of the surface energy of a neck and the elastic energy of a deformed nanoparticle. In the case of small values of the parameter δ ≪ 1, the critical detachment force approaches a critical Johnson, Kendall, and Roberts force, f* ≈ 1.5πWR(p), as is usually the case for strongly cross-linked, large nanoparticles. However, in the opposite limit, corresponding to soft small nanoparticles for which δ≫1, the critical detachment force, f*, scales as f*∝ γ(p)(3/2)R(p)(1/2)G(-1/2). Simulation data are described by a scaling function f*∝ γ(p)(3/2)R(p)(1/2)G(-1/2)δ(-1.89).

Layer-by-Layer Assembly of Charged Nanoparticles on Porous Substrates: Molecular Dynamics Simulations

We performed molecular dynamics simulations of a multilayer assembly of oppositely charged nanoparticles on porous substrates with cylindrical pores. The film was constructed by sequential adsorption of oppositely charged nanoparticles in layer-by-layer fashion from dilute solutions. The multilayer assembly proceeds through surface overcharging after completion of each deposition step. There is almost linear growth in the surface coverage and film thickness during the deposition process. The multilayer assembly also occurs inside cylindrical pores. The adsorption of nanoparticles inside pores is hindered by the electrostatic interactions of newly adsorbing nanoparticles with the multilayer film forming inside the pores and on the substrate. This is manifested in the saturation of the average thickness of the nanoparticle layers formed on the pore walls with an increasing number of deposition steps. The distribution of nanoparticles inside the cylindrical pore was nonuniform with a significant excess of nanoparticles at the pore entrance.

Layer-by-layer assembly of polyelectrolyte chains and nanoparticles on nanoporous substrates: molecular dynamics simulations.

We performed molecular dynamics simulations of a multilayered assembly of oppositely charged polyelectrolyte chains and nanoparticles on porous substrates with cylindrical pores. The film was constructed by the sequential adsorption of oppositely charged species in a layer-by-layer fashion from dilute solutions. The multilayer assembly proceeds through surface overcharging after the completion of each deposition step. The substrate overcharging fraction fluctuates around 0.5 for nanoparticle-polyelectrolyte systems and around 0.4 for polyelectrolyte-polyelectrolyte systems. The surface coverage increases linearly with the number of deposition steps. The rate of surface coverage increases as a function of the number of deposition step changes when the pore is blocked. The closing of the pore occurs from the pore entrance for nanoparticle-polyelectrolyte systems. In the case of polyelectrolyte-polyelectrolyte systems, the pore plug is formed inside the pore and then spreads toward the pore ends.