Naga Rajesh Tummala

SDS surfactants on carbon nanotubes: aggregate morphology.

Although carbon nanotubes have attracted enormous research interest, their practical application is still hindered, primarily, by the difficulty of separating them into samples monodispersed in diameter, chirality, and length. Recent advances show that ultracentrifugating carbon nanotube dispersions stabilized by surfactants is a promising route for achieving the desired separation. For further perfectioning this procedure it is necessary to know how surfactants adsorb on nanotubes of different diameters, which determines the nanotube-surfactant aggregate effective density and the nanotube-nanotube potential of mean force. Because only limited experimental data are available to elucidate these phenomena, we report here an extensive all-atom molecular dynamics study on the morphology of sodium dodecyl sulfate (SDS) surfactant aggregates adsorbed on (6,6), (12,12), and (20,20) single walled carbon nanotubes at room conditions. Our calculations reveal that the nanotube diameter is the primary factor that determines the morphology of the aggregates because of a competition between the entropic and energetic advantage encountered by the surfactants when they wrap one nanotube, and the enthalpic penalty faced during this process due to bending of the surfactant molecule. The data are in qualitative agreement with the neutron scattering results reported by Yurekli et al. [J. Am. Chem. Soc. 2004, 126, 9902], and for the first time provide an atomic-level description helpful in designing better separation, as well as stabilization techniques for aqueous carbon nanotube dispersions.

Stabilization of Aqueous Carbon Nanotube Dispersions Using Surfactants: Insights from Molecular Dynamics Simulations

Techniques for separating bundles of carbon nanotubes into homogeneous dispersion are still under development, although a few methods have been successful at the laboratory scale. Understanding the effective interactions between carbon nanotubes in the presence of dispersing agents will provide the necessary information to develop better methods and also to refine the existing ones. We present here results from all-atom molecular dynamics simulations for aqueous flavin mononucleotide (FMN), which has been found experimentally to efficiently separate single-walled carbon nanotubes (SWNTs) based on diameter and chirality. We report results for the aggregate morphology of FMN on SWNTs of different diameters, as well as the potential of mean force between (6,6) SWNTs in the presence of aqueous FMN. The results are compared to the potential of mean force between SWNTs in aqueous sodium dodecyl sulfate (SDS). Our detailed analysis is used to explain the role of FMN, water, and sodium ions in providing a strong repulsive barrier between approaching SWNTs.

Role of counterion condensation in the self-assembly of SDS surfactants at the water-graphite interface.

The aggregate structure of sodium dodecyl sulfate (SDS) adsorbed at the graphite-water interface has been studied with the aid of molecular dynamics (MD) simulations. As expected, our results show that adsorbed SDS yields hemi-cylindrical micelles. The hemi-cylindrical aggregates in our simulations closely resemble all structural and morphological details provided by previous solution atomic force microscopy (AFM) experiments. More interestingly, our data indicate that SDS head groups do not provide a complete shield to the hydrophobic tails. Instead, we found regions in which the hydrophobic tails are exposed to the aqueous solution. By conducting a parametric study for SDS-like nonionic surfactants we show that electrostatic interactions between SDS head groups and counterions are responsible for the unexpected result. Our interpretation is corroborated by density profiles, analysis of the coordination states, and mean square displacement data for both the adsorbed SDS surfactants and the counterions in solution. Counterion condensation appears to be a physical phenomenon that could be exploited to direct the assembly of advanced nanostructured materials.

C12E6 and SDS surfactants simulated at the vacuum-water interface.

The effect of surface coverage on the aggregate structure for the nonionic hexaethylene glycol monododecyl ether (C(12)E(6)) and anionic sodium dodecyl sulfate (SDS) surfactants at vacuum-water interface has been studied using molecular dynamics simulations. We report the aggregate morphologies and various structural details of both surfactants as a function of surface coverage. Our results indicate that C(12)E(6) tail groups orient less perpendicularly to the vacuum-water interface compared to SDS ones. Interfacial C(12)E(6) shows a transition from gaslike to liquidlike phases as the surface density increases. However, even at the largest coverage considered, interfacial C(12)E(6) aggregates show more disordered structures compared to SDS ones. Both surfactants exhibit a non-monotonic change in planar mobility as the available surface area per molecule varies. The results are interpreted on the basis of the molecular features of both surfactants, with particular emphasis on the properties of the surfactant heads, which are nonionic, long, and flexible for C(12)E(6), as opposed to ionic, compact, and rigid for SDS.

Static and Dynamic Energetic Disorders in the C60, PC61BM, C70, and PC71BM Fullerenes

We use a combination of molecular dynamics simulations and density functional theory calculations to investigate the energetic disorder in fullerene systems. We show that the energetic disorder evaluated from an ensemble average contains contributions of both static origin (time-independent, due to loose packing) and dynamic origin (time-dependent, due to electron-vibration interactions). In order to differentiate between these two contributions, we compare the results obtained from an ensemble average approach with those derived from a time average approach. It is found that in both amorphous C60 and C70 bulk systems, the degrees of static and dynamic disorder are comparable, while in the amorphous PC61BM and PC71BM systems, static disorder is about twice as large as dynamic disorder.

Molecular-Scale Understanding of Cohesion and Fracture in P3HT:Fullerene Blends

Quantifying cohesion and understanding fracture phenomena in thin-film electronic devices are necessary for improved materials design and processing criteria. For organic photovoltaics (OPVs), the cohesion of the photoactive layer portends its mechanical flexibility, reliability, and lifetime. Here, the molecular mechanism for the initiation of cohesive failure in bulk heterojunction (BHJ) OPV active layers derived from the semiconducting polymer poly(3-hexylthiophene) [P3HT] and two monosubstituted fullerenes is examined experimentally and through molecular-dynamics simulations. The results detail how, under identical conditions, cohesion significantly changes due to minor variations in the fullerene adduct functionality, an important materials consideration that needs to be taken into account across fields where soluble fullerene derivatives are used.

Salt-specific effects in aqueous dispersions of carbon nanotubes

Tremendous progress has been made to stabilize carbon nanotube dispersions using surfactants, although many questions await answer to design surfactant formulations that selectively stabilize nanotubes mono-dispersed in diameter and chirality. Stimulated by recent experimental observations [J. Am. Chem. Soc., 2010, 132, 16165–16175], we attempt here to quantify how changing the counter-ion (Cs+ instead of Na+) affects the morphology of dodecyl sulfate surfactants adsorbed on carbon nanotubes. Using atomistic molecular dynamics we simulated aqueous cesium dodecyl sulfate (CsDS) adsorbed on (6,6), (12,12), and (20,20) single-walled carbon nanotubes (SWCNTs) at ambient conditions. When compared to results for sodium dodecyl sulfate (SDS), our results suggest that surface aggregates with Cs+ ions, compared to Na+, yield a more compact coverage of the nanotubes at the surfactant surface coverage of 0.25 nm2 per headgroup, with the surfactant heads extended towards the bulk aqueous solution, and prevent water from accessing the nanotube surface. These morphological results suggest that CsDS should be more effective than SDS at stabilizing aqueous carbon nanotubes dispersions. More importantly, these results were obtained only for the (6,6) nanotubes simulated. For the wider nanotubes our simulations show limited, if any, differences in the morphology of the surfactant aggregates when the Na+ ions are substituted with Cs+ ones. To validate our results we measured experimental UV-Vis-NIR absorbance spectra for aqueous carbon nanotubes with diameters similar to that of (6,6) and of (12,12) nanotubes stabilized by SDS at increasing salt concentration (CsCl vs. NaCl). The results are indicative of changes in the surfactant self-assembled structure on the narrow nanotubes in the presence of Cs+ ions, while data for the wider tubes only suggest salt-screening effects for both Na+ and Cs+ ions. The different salt-specific behavior observed for the surfactants adsorbed on narrow vs. wide carbon nanotubes could be exploited for the selective stabilization of mono-dispersed carbon nanotube samples, although a surfactant more effective than SDS should be used.

Influence of Molecular Shape on Solid-State Packing in Disordered PC61BM and PC71BM Fullerenes

Molecular and polymer packings in pure and mixed domains and at interfacial regions play an important role in the photoconversion processes occurring within bulk heterojunction organic solar cells (OSCs). Here, molecular dynamics simulations are used to investigate molecular packing in disordered (amorphous) phenyl-C70-butyric acid-methyl ester (PC71BM) and its C60 analogue (PC61BM), the two most widely used molecular-based electron-accepting materials in OSCs. The more ellipsoidal character of PC71BM leads to different molecular packings and phase transitions when compared to the more spherical PC61BM. Though electronic structure calculations indicate that the average intermolecular electronic couplings are comparable for the two systems, the electronic couplings as a function of orientation reveal important variations. Overall, this work highlights a series of intrinsic differences between PC71BM and PC61BM that should be considered for a detailed interpretation and modeling of the photoconversion process in OSCs where these materials are used.

Structure-processing-property correlations in solution-processed, small-molecule, organic solar cells

Alkyl chains are often attached to the periphery of semiconductor molecules to impart solubility and they represent a pervasive structural element in solution processable, organic photovoltaics (OPV). It is important to understand the effects of such substitutions on the morphology and performance of organic solar cells. This investigation focuses on determining structure–property correlations in OPV devices constructed with small-molecule, solution processable electron donors based on benzothiadiazole–dithienopyrrole, mixed with the electron acceptor PCBM. Two donor molecules with the same opto-electronic molecular properties but differing alkyl substituents – without (BD) or with (BD6) hexyl side chains – are studied. The resulting device data for fabricated solar cells, across a range of processing conditions, is compared to thin-film morphology, spectroscopy, thermal analysis, and molecular dynamics simulations. Two device states of higher and lower performance, depending on the casting solvent, are obtained for the molecule without the side chains (BD); both states have amorphous mesoscale structure, but show subtle differences in the nanoscale phase separation. In contrast, for the molecule with side chains (BD6) devices have highly variable reproducibility and middling efficiency and photocurrent. The BD6 donor exhibits lower miscibility with PCBM, which correlates with the formation of a donor-enriched layer on the surface of the solar cell.

Hydrogen-bond dynamics for water confined in carbon tetrachloride-acetone mixtures.

In a variety of biological scenarios water is found trapped within hydrophobic environments (e.g., ion channels). Its behavior under such conditions is not well understood and therefore is attracting enormous scientific attention. It is of particular interest to understand how the confining environment affects both the structure and dynamics of water. Within this scenario, we report molecular dynamics simulation results for water trapped in a mixture of acetone and carbon tetrachloride whose composition mimics the one employed in recently reported experiments [Gilijamse, J. J.; et al. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 3202]. We show here that the water molecules dissolved in the carbon tetrachloride-acetone mixture assemble in clusters of varying sizes, that the longevity of hydrogen bonds between confined water molecules strongly depends on the cluster size, and that hydrogen bonds last longer for small water clusters in confined water than they do in bulk water. The simulated FT-IR spectra for the confined water are shifted at longer frequencies compared to those observed for bulk liquid water. We discuss the dependence of the FT-IR spectrum on the size of the water clusters dispersed in the carbon tetrachloride-acetone matrix. We also study in detail the rotational orientation of the dispersed water molecules, and we discuss how the composition of the organic matrix affects the results. By enhancing the interpretation of the experimental data, our results contribute to developing a molecular-based understanding of the relationship between environment and water properties.