Carbon nanotubes and graphene in aqueous surfactant solutions : molecular simulations and theoretical modeling

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2012.Cataloged from PDF version of thesis.Includes bibliographical references (p. 130-148).This thesis describes combined molecular simulations and theoretical modeling studies, supported by experimental observations, on properties and applications of carbon nanotubes (CNTs) and graphene sheets dispersed in aqueous surfactant solutions. In particular, the role of the bile salt anionic surfactant, sodium cholate (SC), in dispersing single-walled carbon nanotubes (SWCNTs) and graphene sheets in aqueous solutions was investigated. In addition, the roles of various surfactants (SC, sodium dodecyl sulfate (SDS, anionic), and cetyl trimethylammonium bromide (CTAB, cationic)) in controlling the extent of functionalization of SWCNTs were investigated. First, the surface structure of adsorbed surfactant (SC) molecules on the SWCNT surface was studied using molecular dynamics (MD) simulations, and the interactions between two SWCNT-SC complexes were determined using potential of mean force (PMF) calculations. I found that the cholate ions wrap around the SWCNT like a ring, and exhibit a small tendency to orient perpendicular to the cylindrical axis of the SWCNT, a unique feature that has not been observed for conventional linear surfactants such as SDS. By comparing my simulated PMF profile of SC with the PMF profile of SDS reported in the literature, I found that, at the saturated surface coverages, SC is a better stabilizer than SDS, a finding that is consistent with the widespread use of SC to disperse SWNTs in aqueous media. Second, I probed the surface structure and electrostatic potential of monolayer graphene dispersed in a SC aqueous solution. Subsequently, I quantified the interactions between two graphene-SC complexes using PMF calculations, which confirmed the existence of a metastable bilayer graphene structure due to the steric hindrance of the confined SC molecules. Interestingly, one faces a dilemma when using surfactants to disperse and stabilize graphene in aqueous solution: on the one hand, surfactants can stabilize graphene aqueous dispersions, but on the other hand, they prevent the formation of new AB-stacked bilayer and trilayer graphene resulting from the reaggregation process. Finally, the lifetime and time-dependent distribution of various graphene layer types were predicted using a kinetic model of colloid aggregation, and each graphene layer type was further decomposed into subtypes, including the AB-stacked species and various turbostratic species. Third, I showed that the free energy of diazonium adsorption onto the SWCNT-surfactant complex, determined using PMF calculations, can be used to rank surfactants (SC, SDS, and CTAB) in terms of the extent of functionalization attained following their adsorption on the nanotube surface. The difference in binding affinities between linear and rigid surfactants was attributed to the synergistic binding of the diazonium ion to the local "hot/cold spots" formed by the charged surfactant heads. A combined simulation-modeling framework was developed to provide guidance for controlling the various sensitive experimental conditions needed to achieve the desired extent of SWCNT functionalization. In conclusion, molecular simulations of the type discussed in this thesis, which can be used to complement traditional continuum-based theories, provide a powerful tool to investigate nano-structured aqueous dispersions. The combined simulation-modeling methodology presented in this thesis can be extremely useful in predicting material properties and optimizing experimental procedures in order to minimize tedious and time-consuming trial-and-error experimentation when studying other nanoscale systems of interest.by Shangchao Lin.Ph.D

A computer simulation and molecular-thermodynamic framework to model the micellization of ionic branched surfactants in aqueous solution

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2008.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Includes bibliographical references (leaves 115-132).Surfactants, or surface active agents, are chemicals exhibiting amphiphilic behavior toward a solvent. This amphiphilic character leads to increased activity at interfaces and to self-assembly into micellar aggregates beyond a threshold surfactant concentration, referred to as the critical micelle concentration (CMC), in bulk solutions. As a result of these unique attributes, surfactants are used in many pharmaceutical, industrial, and environmental applications, including biological separations, fat metabolism during digestion, drug delivery, and water purification. Selection of the appropriate surfactant for a given application is often motivated by the need to control bulk solution micellization properties, such as the CMC and the micelle shape and size. The ability to make molecular-level predictions of these surfactant properties would allow formulators in industry to speed up the design and optimization of new surfactant formulations. In this thesis, a combined computer simulation/molecular-thermodynamic (CS-MT) modeling approach was developed and utilized to study the micellization behavior of ionic branched surfactants, which are a class of surfactants of great industrial relevance in applications such as detergency, emulsification, and enhanced-oil recovery. In the CSMT modeling approach, molecular dynamics (MD) simulations are used to obtain input parameters for molecular-thermodynamic (MT) modeling of surfactant micellization.This approach is motivated by the limitations inherent in computer simulations (the high computational expense associated with modeling self-assembly) and in MT modeling approaches (their restriction to structurally and chemically simple surfactants). One key input required for traditional MT modeling is the identification of the hydrated ("head") and the dehydrated ("tail") portions of surfactants in a self-assembled micellar aggregate. Using the results of MD simulations of surfactants in a micellar environment, a novel head and tail identification method was developed based on the determination of a conceptual micelle core-water interface. The introduction of an interfacial region consisting of partially hydrated, neutral atomic groups required formulating an improved surfactant tail packing approach.(cont.) Another key input required in the CS-MT modeling approach is the fractional degree of hydration of each atomic group in the ionic branched surfactants considered in this thesis, which can be used to accurately quantify the hydrophobic driving force for micelle formation in aqueous media. Fractional hydration profiles were obtained by conducting two MD simulations, one in a bulk water environment and the other in a micellar environment. By investigating the radial distribution function (RDF) between each surfactant group and hydrating atoms which are capable of forming hydrogen-bonds and coordinate-bonds, an updated cutoff distance for counting hydrating contacts was selected. These simulated fractional hydration profiles were then utilized as inputs in the MT model, which enables calculation of the minimum free energy associated with micelle formation, from which the CMC and the optimal micelle shape and size can be predicted at the molecular level. The MD simulations were shown to extend the applicability of the traditional MT modeling approach to more complex surfactant systems than had been possible to date. A rich variety of ionic branched surfactants were modeled using the new CS-MT modeling approach, including two homologous series of simple secondary alkyl sulfonates and three classes of more complex ionic branched surfactants possessing aromatic moieties. For each of the ionic branched surfactants modeled, the predictions of the CS-MT modeling approach were found to be in reasonable agreement with the experimental data, including accounting for the chemical and structural complexities of the branched surfactants more accurately. The CS-MT modeling approach developed in this thesis not only extends our ability to make accurate molecular-level predictions of the micellization behavior of complex surfactants, but it also contributes to our overall fundamental understanding of the solution behavior of surfactants.by Shangchao Lin.S.M

Thermal Imprint Introduced Crystallization of A Solution Processed Subphthalocyanine Thin Film

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/134175/1/admi201600179.pd

Enhancing thermoelectric properties of isotope graphene nanoribbons via machine learning guided manipulation of disordered antidots and interfaces

Structural manipulation at the nanoscale breaks the intrinsic correlations among different energy carrier transport properties, achieving high thermoelectric performance. However, the coupled multifunctional (phonon and electron) transport in the design of nanomaterials makes the optimization of thermoelectric properties challenging. Machine learning brings convenience to the design of nanostructures with large degree of freedom. Herein, we conducted comprehensive thermoelectric optimization of isotopic armchair graphene nanoribbons (AGNRs) with antidots and interfaces by combining Green's function approach with machine learning algorithms. The optimal AGNR with ZT of 0.894 by manipulating antidots was obtained at the interfaces of the aperiodic isotope superlattices, which is 5.69 times larger than that of the pristine structure. The proposed optimal structure via machine learning provides physical insights that the carbon-13 atoms tend to form a continuous interface barrier perpendicular to the carrier transport direction to suppress the propagation of phonons through isotope AGNRs. The antidot effect is more effective than isotope substitution in improving the thermoelectric properties of AGNRs. The proposed approach coupling energy carrier transport property analysis with machine learning algorithms offers highly efficient guidance on enhancing the thermoelectric properties of low-dimensional nanomaterials, as well as to explore and gain non-intuitive physical insights

Aggregation-induced emission in lamellar solids of colloidal perovskite quantum wells

The outstanding excitonic properties, including photoluminescence quantum yield (ηPL), of individual, quantum-confined semiconductor nanoparticles are often significantly quenched upon aggregation, representing the main obstacle toward scalable photonic devices. We report aggregation-induced emission phenomena in lamellar solids containing layer-controlled colloidal quantum wells (QWs) of hybrid organic-inorganic lead bromide perovskites, resulting in anomalously high solid-state ηPL of up to 94%. Upon forming the QW solids, we observe an inverse correlation between exciton lifetime and ηPL, distinct from that in typical quantum dot solid systems. Our multiscale theoretical analysis reveals that, in a lamellar solid, the collective motion of the surface organic cations is more restricted to orient along the [100] direction, thereby inducing a more direct bandgap that facilitates radiative recombination. Using the QW solids, we demonstrate ultrapure green emission by completely downconverting a blue gallium nitride light-emitting diode at room temperature, with a luminous efficacy higher than 90 lumen W−1 at 5000 cd m−2, which has never been reached in any nanomaterial assemblies by far.ISSN:2375-254

Sustainable Neighbourhood Reconstruction in the Urban District

The neighbourhood reconstruction involves three key stakeholders—municipality, builders and residents. There are some conflicts among them due to their different standpoints, and the reasons of most conflicts relate to human needs of residents when they are not met. The main purpose of this study is to create a general socially sustainable neighbourhood reconstruction process through the lens of FSSD and Max-Neef‘s human needs theory. This new process will promote the collaboration among these three key stakeholders and help residents to have their basic human needs fulfilled. First of all, a vision of a sustainable neighbourhood reconstruction process is created and amended based upon literature review and authorities‘ feedback; then a summary of the current reality of the reconstruction process is addressed in light of a case study; followed by the analysis of the gap between vision and reality. At last, different suggestions are given with the purpose of eliminating the gap

Does family business mean family operation? The study of the relationship between ownership and managerial authority in the transition period of Chinese family business

Family businesses play a very important role in the world’s economy. Chinese family businesses grow quickly after the reforming and opening of China’s economy since 1978. We focus on the management problems, the relationship between ownership and managerial authority in small and medium-sized family businesses which play an important role, especially in the southeast coastal areas of China. From now on is the peak period of China’s first generation of family business transfer to the second generation successor and family management deficiencies have become increasingly prominent in the further development and enlarge process. It is good time to separate the ownership and managerial authority, employ professional managers.But there are also have some problems and difficulties during this process.The main content of the thesis is “why”, “how” and “what” about the issue of separation the ownership and managerial authority, employ professional managers.We focus on the reasons and difficulties to separate. And how does family business operate in the pattern of professional manage.At last we will get the conclusion and point out some recommendation

Sustainable Neighbourhood Reconstruction in the Urban District

The neighbourhood reconstruction involves three key stakeholders—municipality, builders and residents. There are some conflicts among them due to their different standpoints, and the reasons of most conflicts relate to human needs of residents when they are not met. The main purpose of this study is to create a general socially sustainable neighbourhood reconstruction process through the lens of FSSD and Max-Neef‘s human needs theory. This new process will promote the collaboration among these three key stakeholders and help residents to have their basic human needs fulfilled. First of all, a vision of a sustainable neighbourhood reconstruction process is created and amended based upon literature review and authorities‘ feedback; then a summary of the current reality of the reconstruction process is addressed in light of a case study; followed by the analysis of the gap between vision and reality. At last, different suggestions are given with the purpose of eliminating the gap

Lithiation-Assisted Strengthening Effect and Reactive Flow in Bulk and Nanoconfined Sulfur Cathodes of Lithium–Sulfur Batteries

Lithiation of electrode materials can lead to significant microstructural evolution and changes in their mechanical behaviors in lithium batteries. Lithium–sulfur (Li–S) batteries have recently attracted extensive attention, where carbon matrices have been utilized to retain S content by restricting the dissolution of polysulfide into electrolytes. Here we systematically investigate S cathode upon unconfined and nanoconfined lithiation using reactive molecular dynamics simulations. We demonstrate the great ductility of lithiated amorphous S cathode (a-Li_<i>x</i>S) governed by overcoordination sites, as well as the resulting strengthening effect of a-Li_<i>x</i>S due to the formation of stronger Li–S bonds upon lithiation. Fracture and cavitation studies also indicate the dominant role of shear banding, which is facilitated by overcoordinated S “plastic carriers”, in accommodating the plastic deformation of a-Li_<i>x</i>S under tensile loading. Based on a chemo-mechanical yield function, we confirm two-dimensionally nanoconfined lithiation reaction can facilitate the out-of-plane inelastic deformation (“reactive flow”) of a-Li_<i>x</i>S at a much lower level of biaxial stress. The atomistic understanding of lithiation behaviors of S cathodes provides fundamental insight into the optimal design of carbon-based S composite cathode with outstanding mechanical integrity, as well as the prediction of lithiation behavior of other electrode materials, such as silicon, metal oxides, and graphite