Zhenli Zhang

A precise packing sequence for self-assembled convex structures.

Molecular simulations of the self-assembly of cone-shaped particles with specific, attractive interactions are performed. Upon cooling from random initial conditions, we find that the cones self-assemble into clusters and that clusters comprised of particular numbers of cones (e.g., 4–17, 20, 27, 32, and 42) have a unique and precisely packed structure that is robust over a range of cone angles. These precise clusters form a sequence of structures at specific cluster sizes (a “precise packing sequence”) that for small sizes is identical to that observed in evaporation-driven assembly of colloidal spheres. We further show that this sequence is reproduced and extended in simulations of two simple models of spheres self-assembling from random initial conditions subject to convexity constraints, including an initial spherical convexity constraint for moderate- and large-sized clusters. This sequence contains six of the most common virus capsid structures obtained in vivo, including large chiral clusters and a cluster that may correspond to several nonicosahedral, spherical virus capsids obtained in vivo. Our findings suggest that this precise packing sequence results from free energy minimization subject to convexity constraints and is applicable to a broad range of assembly processes.

Hydrodynamics and microphase ordering in block copolymers: Are hydrodynamics required for ordered phases with periodicity in more than one dimension?

We use Brownian dynamics (BD), molecular dynamics, and dissipative particle dynamics to study the phase behavior of diblock copolymer melts and to determine if hydrodynamics is required in the formation of phases with greater than one-dimensional periodicity. We present a phase diagram for diblock copolymers predicted by BD and provide a relationship between the inverse dimensionless temperature epsilon/k(B)T and the Flory-Huggins chi parameter, allowing for a quantitative comparison between methods and to mean field predictions. Our results concerning phase behavior are in good qualitative agreement with the theoretical predictions of Matsen and Bates [M. W. Matsen and F. S. Bates, Macromolecules 29, 1091 (1996)]; however, fluctuation effects arising from finite polymer lengths substantially alter the phase boundaries. Our results pertaining to the hydrodynamics are in contrast to earlier work by Groot et al. [R. D. Groot, T. J. Madden, and D. J. Tildesley, J. Chem. Phys. 110, 9739 (1999); D. Frenkel and B. Smit, Understanding Molecular Simulation, 2nd ed. (Academic, New York, 2001)]. In particular, we obtain the hexagonal ordered cylinder phase with BD, a method that does not include hydrodynamics.

Self-assembly of end-tethered nanorods in a neat system and role of block fractions and aspect ratio

We report a computational study of the self-assembly of end-tethered nanorods in a neat system (no solvent). We present morphological phase diagrams for low and moderate aspect ratio rods as a function of inverse temperature vs. relative tether fraction. Our simulations predict that the end-tethered rods self-assemble into hexagonally arranged chiral cylinders, hexagonally perforated lamellae, monolayer and bilayer arrowhead structures and wavy lamellae. For high aspect ratio tethered nanorods and small tether fractions, we observe that the tethered nanorods self-assemble into smectic and zig-zag lamellar morphologies.

Molecular simulation study of self-assembly of tethered V-shaped nanoparticles.

We use Brownian dynamics to investigate the self-assembly of single end tethered, laterally tethered, and double end tethered V-shaped nanoparticles. The simulation results are compared with model bent-core molecules without tethers and polymer tethered nanorods to elucidate the combined effects of V-shaped geometry and the immiscibility between the V-shaped nanoparticles and the tethers on the self-assembled structures. We show that the V-shaped geometry significantly alters the phase diagram of tethered nanoparticles and further that the immiscibility between particles and tethers leads to structures not previously predicted for bent-core molecules. Examples of mesophases predicted include honeycomb, hexagonally packed cylinders, and perforated lamellar phases.

Multi-diagnostic comparison of femtosecond and nanosecond pulsed laser plasmas

Understanding and fully characterizing highly dynamic and rapidly streaming laser ablation plasmas requires multiple techniques for monitoring effects at different stages. By combining multiple diagnostic methods, it is possible to analyze the broad time window over which these ablation plasmas develop and to learn more about the related physical processes that occur. Two laser sources, an 80 fs Ti:Sapphire laser ~780 nm! and a6n sNd:YAG laser ~1.06 mm!, are used in this work in order to compare pulse duration effects at similar wavelengths. Characteristics of the plasma produced by these two lasers are compared under conditions of comparable ablation flux. Results are presented involving correlation of time-resolved Langmuir probe data and electrostatic energy analysis for aluminum plasmas as a representative investigation for metallic systems. In addition, continuous-wave refractive index laser beam deflection is used to characterize the plasma and hot gas generated from boron nitride targets in terms of their ion and neutral atom densities. A self-similarity plasma expansion model is used to analyze the plumes under various conditions. Fundamental data obtained in this way can be relevant to laser micro-machining, laser induced breakdown spectroscopy, and pulsed laser deposition.

Nitride film deposition by femtosecond and nanosecond laser ablation in low-pressure nitrogen discharge gas

Abstract Thin films of TiN and BN are deposited by femtosecond and nanosecond laser ablation in a vacuum chamber with N2 gas discharge at 0.8 mTorr. Use of these activated gas conditions guarantees stoichiometric incorporation of nitrogen in the films. Properties of the deposited films are compared for the two different laser pulse durations. Ultrafast (fs) pulses are at a wavelength of 780 nm and the nanosecond pulses at 355 nm. The film growth is monitored with in situ RHEED, which provides information on the crystal quality of the films during growth. In addition to single-layer films of TiN being grown on silicon, a superlattice of BN/TiN was also fabricated. The TiN films were observed to form first as cubic phase single-crystal material that converted to polycrystal as the film thickness increases. These polycrystals exhibited textured orientation for the nanosecond pulsed depositions but were a randomly oriented fine-grained structure for the femtosecond pulses. The alternating multi-layers of TiN/BN exhibited interesting features that were complicated by surface roughness in the film. However, cross-sectional HRTEM demonstrated that a region of cubic phase BN was present in between two of the cubic phase TiN layers. It is thought that this results from a domain epitaxial relationship between the two materials. Such behavior is expected to be of great potential interest in the fabrication of uniform c-BN films by epitaxial growth.

Generation of massive entanglement through an adiabatic quantum phase transition in a spinor condensate.

We propose a method to generate massive entanglement in a spinor Bose-Einstein condensate from an initial product state through an adiabatic sweep of the magnetic field across a quantum phase transition induced by competition between the spin-dependent collision interaction and the quadratic Zeeman effect. The generated many-body entanglement is characterized by the experimentally measurable entanglement depth in the proximity of the Dicke state. We show that the scheme is robust to practical noise and experimental imperfection and under realistic conditions it is possible to generate genuine entanglement for hundreds of atoms.

Critical density effects in femtosecond ablation plasmas and consequences for high intensity pulsed laser deposition

Abstract Laser ablation plumes produced by a single pulse from an ultrafast laser consist, in the far field where film deposition occurs, of mostly neutral atoms, a percentage of ionized species, and, very often, condensed clusters. In certain situations adding energy to the plume may be of interest for a deposition, and methods for increasing the charged fraction need to be considered. This paper examines these issues and demonstrates a method for overcoming the plasma critical density limitations encountered for absorption of a single pulse. Precisely controlled time-delayed secondary pulses are used to change the average charge state, temperature, and plasma density of the far field plume, with implications for thin film deposition and nano-cluster formation. A plasma-jet nozzle effect is proposed to explain condensed cluster formation of germanium. Results are also presented in relation to enhanced isotope enrichment for boron.

Self-Organization of Nanoscopic Building Blocks into Ordered Assemblies

We studied the self-assembly of nanoscopic building blocks comprised of polymer-tethered nanoparticles using computer simulation and predict that these building blocks can assemble into mono- and multi-layer sheets and shells. The simulations further demonstrate that for some nanoparticle geometries and tethered nanoparticle topologies, ideas from block copolymers, surfactants and liquid crystals can be used to predict the ordered morphologies attained via self- assembly and that for specific cases the morphologies are consistent with Israelachvili packing rules.

Teaching Computational Materials Science for Nanoscale Science and Engineering

We describe the development of a graduate level course designed to teach computational materials science and its application to nanoscale science and engineering. We discuss the use of MatDL, a web-based digital library and materials science resource, as a collaborative learning tool within the context of the course.