Effect of Nanoscale Confinement on the Structure and Properties of Single Conjugated Polymer Molecules

The research presented in this thesis was motivated by questions on the effect of nanoscale confinement on molecular conformation and related photophysical properties of conjugated polymers. Using microdroplet techniques as a method of isolating single molecules of various poly[phenylene vinylenes] we discovered that poly[2-methoxy-5- (2’-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) and poly[2-Methoxy-5-(2\u27- ethylhexyloxy)-1,4-(cyanovinylene)phenylene] (CN-PPV) can be deposited on precleaned glass substrates with unique transition moment orientation. Structural investigation using a combination of fluorescence emission pattern imaging, atomic force microscopy and polarization anisotropy measurements revealed that individual polymer nanostructures had a high degree of intra-molecular with the long axis of the conjugated segment oriented perpendicular to the substrate. The high degree of internal structural order within individual polymer chains affected the spectral and photophysical properties compared to the bulk polymer. The photochemical stability of z-oriented nanostructures was orders of magnitude higher with ≈30x times more photon count rates than the in- plane oriented species in ambient conditions at similar excitation conditions. Z-oriented nanostructures showed narrow bandwidth spectral emission, which was typically redshifted with respect to the bulk polymer spectra. Investigation of the central frequency distribution of the fluorescence emission spectra of MEH-PPV revealed discrete emission from localized conjugated segments within the nanoparticle. Definitive evidence of single site emission from z-oriented polymer nanostructures was obtained from photon correlation measurements. Fluorescence lifetime and fluorescence quantum yield measurements also point to a transition dipole surrounded by a nanoscale dielectric-in these case-conjugated segments in the polymer chain. With facile sample preparation, high photon count rates and high photochemical stability in the ambient conditions and highly pure single photon emission, z-oriented nanostructures can potentially be used as a source for single photon emission for quantum information processing

Nanoscale Fluid Transport: Size and Rate Effects

The transport behavior of water molecules inside a model carbon nanotube is investigated by using nonequilibrium molecular dynamcis (NMED) simulations. The shearing stress between the nanotube wall and the water molecules is identified as a key factor in determining the nanofluidic properties. Due to the effect of nanoscale confinement, the effective shearing stress is not only size sensitive but also strongly dependent on the fluid flow rate. Consequently, the nominal viscosity of the confined water decreases rapidly as the tube radius is reduced or when a faster flow rate is maintained. An infiltration experiment on a nanoporous carbon is performed to qualitatively validate these findings

Effect of Nanoscale Confinement on Glass Transition of Polystyrene Domains from Self-assembly of Block Copolymers

The understanding of size-dependent properties is key to the implementation of nanotechnology. One controversial and unresolved topic is the influence of characteristic size on the glass transition temperature (T(g)) for ultrathin films and other nanoscale geometries. We show that T(g) does depend on size for polystyrene spherical domains with diameters from 20 to 70 nm which are formed from phase separation of diblock copolymers containing a poly(styrene-co-butadiene) soft block and a polystyrene hard block. A comparison of our data with published results on other block copolymer systems indicates that the size dependence of T(g) is a consequence of diffuse interfaces and does not reflect an intrinsic size effect. This is supported by our measurements on 27 nm polystyrene domains in a styrene-isobutylene-styrene triblock copolymer which indicate only a small T(g) depression (3 K) compared to bulk behavior. We expect no effect of size on T(g) in the limit as the solubility parameters of the hard and soft blocks diverge from each other. This strongly segregated limiting behavior agrees with published data for dry and aqueous suspensions of small polystyrene spheres but is in sharp contrast to the strong influence of film thickness on T(g) noted in the literature for free standing ultrathin polystyrene films

Topological transitions in carbon nanotube networks via nanoscale confinement

Efforts aimed at large-scale integration of nanoelectronic devices that exploit the superior electronic and mechanical properties of single-walled carbon nanotubes (SWCNTs) remain limited by the difficulties associated with manipulation and packaging of individual SWNTs. Alternative approaches based on ultra-thin carbon nanotube networks (CNNs) have enjoyed success of late with the realization of several scalable device applications. However, precise control over the network electronic transport is challenging due to i) an often uncontrollable interplay between network coverage and its topology and ii) the inherent electrical heterogeneity of the constituent SWNTs. In this letter, we use template-assisted fluidic assembly of SWCNT networks to explore the effect of geometric confinement on the network topology. Heterogeneous SWCNT networks dip-coated onto sub-micron wide ultra-thin polymer channels exhibit a topology that becomes increasingly aligned with decreasing channel width and thickness. Experimental scale coarse-grained computations of interacting SWCNTs show that the effect is a reflection of an aligned topology that is no longer dependent on the network density, which in turn emerges as a robust knob that can induce semiconductor-to-metallic transitions in the network response. Our study demonstrates the effectiveness of directed assembly on channels with varying degrees of confinement as a simple tool to tailor the conductance of the otherwise heterogeneous network, opening up the possibility of robust large-scale CNN-based devices.Comment: 4 pages, 3 figure

Impact of an Irreversibly Adsorbed Layer on Local Viscosity of Nanoconfined Polymer Melts

We report the origin of the effect of nanoscale confinement on the local viscosity of entangled polystyrene (PS) films at temperatures far above the glass transition temperature. By using marker x-ray photon correlation spectroscopy with gold nanoparticles embedded in the PS films prepared on solid substrates, we have determined the local viscosity as a function of the distance from the polymer-substrate interface. The results show the impact of a very thin adsorbed layer ( 7 nm in thickness) even without specific interactions of the polymer with the substrate, overcoming the effect of a surface mobile layer at the air-polymer interface and thereby resulting in a significant increase in the local viscosity as approaching the substrate interface.T. K. acknowledges the financial support from NSF Grant No. CMMI-084626. Uses of the Advanced Photon Source and the National Synchrotron Light Source were supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contracts No. DE-AC02- 06CH11357 and No. DE-AC02-98CH10886, respectively

Universal behavior of highly-confined heat flow in semiconductor nanosystems: from nanomeshes to metalattices

Nanostructuring on length scales corresponding to phonon mean free paths provides control over heat flow in semiconductors and makes it possible to engineer their thermal properties. However, the influence of boundaries limits the validity of bulk models, while first principles calculations are too computationally expensive to model real devices. Here we use extreme ultraviolet beams to study phonon transport dynamics in a 3D nanostructured silicon metalattice with deep nanoscale feature size, and observe dramatically reduced thermal conductivity relative to bulk. To explain this behavior, we develop a predictive theory wherein thermal conduction separates into a geometric permeability component and an intrinsic viscous contribution, arising from a new and universal effect of nanoscale confinement on phonon flow. Using both experiments and atomistic simulations, we show that our theory is valid for a general set of highly-confined silicon nanosystems, from metalattices, nanomeshes, porous nanowires to nanowire networks. This new analytical theory of thermal conduction can be used to predict and engineer phonon transport in boundary-dominated nanosystems, that are of great interest for next-generation energy-efficient devices

Rhodium nanoflowers stabilized by a nitrogen-rich PEG-tagged substrate as recyclable catalyst for the stereoselective hydrosilylation of internal alkynes

Morphology and size controllable rhodium nanoparticles stabilized by a nitrogen-rich polyoxyethylenated derivative have been prepared by reduction of RhCl3 with NaBH4 in water at room temperature and fully characterized. The flower-like Rh NPs are effective and recyclable catalysts for the stereoselective hydrosilylation of challenging internal alkynes and diynes, affording the (E)-vinylsilanes in quantitative yields for a wide range of substrates. The insolubility of the nanocatalyst in diethyl ether allows its easy separation and recycling

Charge relaxation dynamics of an electrolytic nanocapacitor

Understanding ion relaxation dynamics in overlapping electric double layers (EDLs) is critical for the development of efficient nanotechnology based electrochemical energy storage, electrochemomechanical energy conversion and bioelectrochemical sensing devices as well as controlled synthesis of nanostructured materials. Here, a Lattice Boltzmann (LB) method is employed to simulate an electrolytic nanocapacitor subjected to a step potential at t = 0 for various degrees of EDL overlap, solvent viscosities, ratios of cation to anion diffusivity and electrode separations. The use of a novel, continuously varying and Galilean invariant, molecular speed dependent relaxation time (MSDRT) with the LB equation recovers a correct microscopic description of the molecular collision phenomena and enhances the stability of the LB algorithm. Results for large EDL overlaps indicated oscillatory behavior for the ionic current density in contrast to monotonic relaxation to equilibrium for low EDL overlaps. Further, at low solvent viscosities and large EDL overlaps, anomalous plasma-like spatial oscillations of the electric field were observed that appeared to be purely an effect of nanoscale confinement. Employing MSDRT in our simulations enabled a modeling of the fundamental physics of the transient charge relaxation dynamics in electrochemical systems operating away from equilibrium wherein Nernst-Einstein relation is known to be violated.Comment: Accepted for publication in the Journal of Physical Chemistry C on October 30 2014. Supplementary info available free of charge via the Internet at http://pubs.acs.org. Revised version includes more details on the computation of the molecular speed dependent relaxation time (MSDRT) and emphasizes the Galilean invariance of the computed MSDR