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Nanodroplets, the basis of complex and advanced nanostructures such as quantum rings, quantum dots and quantum dot clusters for future electronic and optoelectronic materials and devices, have attracted the interdisciplinary interest of chemists, physicists and engineers. This book combines experimental and theoretical analyses of nanosized droplets which reveal many attractive properties. Coverage includes nanodroplet synthesis, structure, unique behaviors and their nanofabrication, including chapters on focused ion beam, atomic force microscopy, molecular beam epitaxy and the "vapor-liquid- solid" route. Particular emphasis is given to the behavior of metallic nanodroplets, water nanodroplets and nanodroplets in polymer and metamaterial nanocomposites. The contributions of leading scientists and their research groups will provide readers with deeper insight into the chemical and physical mechanisms, properties, and potential applications of various nanodroplets.

Inhaltsverzeichnis

Frontmatter

Chapter 1. Generation of Nanodroplets and Its Applications

The generation mechanism of charged nanodroplets is explained here using Thomson theory for ion-induced nucleation. Also described are the charged nanodroplet generator (CNDG) based on this theory and the method to measure the size of charged nanodroplets. The CNDG can generate charged nanodroplets with a geometric mean diameter

D

pg

= 1.3–1.8 nm for TEOS and

D

pg

= 1.3 nm for H

2

O in O

2

gas and with

D

pg

= 5.0–5.5 nm for Co(CO)

3

NO in H

2

gas. Four successful applications of these charged nanodroplets are also described as the following: (1) synthesis of non-agglomerated SiO

2

nanoparticles with a diameter smaller than 10 nm, (2) patterning on a substrate by selectively depositing nanoparticles onto a charged line pattern, (3) fabrication of a high-performance magnetoresistance device, and (4) sterilization of bacteria (yeast fungi and

Escherichia coli

) by negatively charged H

2

O nanodroplets.

Motoaki Adachi, Takuya Kinoshita

Chapter 2. Nanodroplet Formations in Electrospun Fibers of Immiscible Polymer Blends and Their Effects on Fractionated Crystallization

A new and facile method to obtain polymeric nanodroplets was developed by thermally annealing of electrospun fibers of immiscible polymer blends. Through thermally annealing at a temperature slightly above the glass transition temperature of the matrix, the ribbon- or fiber-like dispersed phase broke up into nanodroplet with the diameter mainly in the range of 50–300 nm due to the Plateau-Rayleigh instability. Our study shows that these nanodroplets can be used to study the fractionated crystallization and homogeneous nucleation of almost any semicrystalline polymers, such as poly(ethylene oxide), poly(vinylidene fluoride), polyethylene, and polypropylene. We observed that the homogeneously nucleated crystallization of PVDF took place at 55–60°C for the first time. Additionally, this method can be utilized to investigate the effect of nanoconfinement on crystalline morphologies of semicrystalline polymers.

Ganji Zhong, Lei Zhu, Hao Fong

Chapter 3. Dynamic Study of Nanodroplet Nucleation and Growth Using Transmitted Electrons in ESEM

Experimental methods for wettability research are reviewed. A novel method for quantitative wettability study at nanoscale is presented. It is based on measuring transmitted electrons through nanodroplets using wet scanning transmission electron microscope (wet-STEM) detector in environmental scanning electron microscope (ESEM). The quantitative information of the nanodroplet shape and contact angle is obtained by fitting Monte Carlo simulation results for transmitted electrons with experimental results. Dynamic in situ imaging has showed that irregularities at the water film boundaries constituted nucleation sites for both dropwise and filmwise condensation. The initial stages of nucleation, growth, and coalescence have been studied as well as the growth power law dependence.

Zahava Barkay

Chapter 4. Self-Assembly of Nanodroplets in Nanocomposite Materials in Nanodroplets Science and Technology

Use of metal nanoarchitectures is increasing in electronics, diagnostics, therapeutics, sensing, and microelectromechanical systems due to their unique electromagnetic and physicochemical properties. This chapter examines physical, chemical, and hybrid methods to assemble metal nanodroplets in single- and multidimensional geometries and phases. Reductive self-assembly offers a route to economic, scale-able preparation of nanodroplets and stabilization on solid substrates that could lead to atom-level tune-ability. Enhanced control and real-time characterization have been used to uncover thermodynamic and transport mechanisms of nanodroplet self-assembly to enhance prediction and control of morphological features. Physicochemical principles of reductive nanodroplet self-assembly are examined to provide a framework to modulate local surface forces and control orderly self-assembly of metallic nanostructures.

D. Keith Roper

Chapter 5. Ordering of Ga Nanodroplets by Low-Energy Ion Sputtering

Ordered nanostructures have attracted much attention due to their potential in realizing novel device applications. In this chapter, we present a study of ordered nanodroplets fabricated by low-energy ion sputtering on GaAs surfaces. The morphological evolution of a GaAs (001) surface exposed to a Ga

+

focused ion beam was investigated as a function of beam energy, incidence, current, sputter time, and dwell time. The sputter yield of the target, surface roughness, amount of material deposited on the surface, and temperature of the substrate were evaluated. The experimental results show formation of self-assembled Ga metal droplets. Control over the size, density, and ordering of the droplets is possible for various sets of ion beam parameters. The arrays of ordered nanodroplets have potential application as templates as well as a local nanosource in molecular beam epitaxy for fabricating ordered semiconductor structures such as quantum dots.

Sabina Koukourinkova, Zhiming M. Wang, Jiang Wu, Xingliang Xu, Mourad Benamara, Peter Moeck, Gregory J. Salamo

Chapter 6. Atomistic Mechanisms Underlying the Freezing Behavior of Metal Nanodroplets

The freezing behavior of nanometer-sized particles of metallic systems is still an open issue, with considerable relevance to a wide spectrum of industrial applications. Understanding the fundamental mechanisms underlying the liquid-to-solid phase transition represents one of the necessary achievements to enable a definite progress for both science and engineering. Aimed at providing a general overview of the molecular dynamics methods that can be used to profitably investigate the response of these systems to a decrease of temperature, this chapter focuses on the phase transition behavior of Au and Ag droplets. In the idea of discussing specific cases as closest as possible to real ones, the control of temperature has been performed by using a collisional thermostat. It is shown that unsupported droplets on the order of a few nanometers in radius can exhibit a relatively complicated dynamics as the freezing point is approached.

Francesco Delogu

Chapter 7. Dynamics of Nanodroplets on Structured Surfaces

Fluids on the nanoscale behave qualitatively different from macroscopic systems. This becomes particularly evident if a free liquid–liquid or liquid–gas interface is close to a solid surface such as in the case of nanodroplets. In contrast to macroscopic drops, hydrodynamic slip, thermal fluctuations, the molecular structure of the liquid, and the range of the intermolecular interactions are important for the structure and the dynamics of such open nanofluidic systems. After a review of the macroscopic modeling and behavior of nonvolatile droplets on structured substrates, we discuss the static and dynamic peculiarities on the nanoscale with special emphasis on theory. In particular we show that nanodroplets experience long-ranged lateral interactions with sharp surface features and that their free energy might be lower on a less wettable part of the substrate surface. A discussion of possible experiments for observing these phenomena is followed by a summary and an outlook.

Markus Rauscher

Chapter 8. Atomistic Simulation of Nanodroplet Collisions with a Wall: Fragmentation and Impact Desolvation of Macromolecules

Impacts of nanodroplets on a hard wall are studied using molecular-dynamics simulation. We focus on water droplets; both pure solvent droplets and droplets filled with a macromolecule are investigated. By choosing a hydrophilic (polyketone) and a hydrophobic (polyethylene) polymer, the effects of the water–polymer interaction can be studied. The process of droplet fragmentation and the ensuing isolation of the embedded macromolecule are investigated. The energy and time dependence of the process is analyzed for various droplet–polymer combinations. By changing droplet size, polymer size, and polymer species separately, we can assess the influence of these factors individually. We demonstrate that the ratio of the impact energy,

E

, to the cohesive energy,

E

coh

, of the droplet is the key quantity characterizing the droplet fragmentation process. If the impact energy per molecule

$$E < (0.29\text{-}0.4) \cdot E_{\mathrm{coh}}$$

, the droplet is reflected without fragmenting. Beyond that impact energy fragmentation of the droplet abruptly starts. At

E

=

E

coh

, the fragmentation process already results in a fine dispersal of the droplet into daughter droplets. The disintegration process continuously increases with collision energy. We find that the polymer is isolated for impact energies

E

per solvent molecule, which exceed a threshold value of the order of the cohesive energy

E

coh

of the solvent. We find that in this energy regime, the temperature of the isolated polymer increases linearly with

E

.

Herbert M. Urbassek, Si Neng Sun

Chapter 9. Polymer Films with Nanosized Liquid-Crystal Droplets: Extinction, Polarization, Phase, and Light Focusing

Extinction and polarization state of light transmitted through a polymer-dispersed liquid-crystal (PDLC) film with nanosized, spherical, nonabsorbing nematic droplets is investigated theoretically. Scattering properties of a single droplet are described by the Rayleigh–Gans approximation. Propagation of coherent light field is described in the frame of the Foldy–Twersky theory. The concept of multilevel order parameters is used. Equations for extinction coefficients, phase shift, and polarization of transmitted light for layers with random and oriented droplets are written and discussed. Conditions for circular and linear polarization of light are determined and investigated. Polarization-independent focusing of light by films made of PDLC is considered. The results are in good coincidence with the known experimental data.

Valery A. Loiko

Chapter 10. Clusters and Nanoparticles in Superfluid Helium Droplets: Fundamentals, Challenges and Perspectives

Helium nanodroplets provide a cold and confined environment that offers many possibilities for the formation and investigation of clusters and nanoparticles. Here we present a review describing the fundamental properties of helium droplets and address in particular their application to and importance in the study of clusters and nanoparticles. We highlight several key experiments on atomic and molecular clusters and then turn our attention to very recent work on using helium droplets for nanoparticle synthesis. Finally, we look to the future and consider some areas where the growth of new nanoparticles via this route may be beneficial.

Shengfu Yang, Andrew M. Ellis

Chapter 11. Reactive Dynamics in Confined Water by Reversed Micelles

The excited state reactive dynamics of the fluorescence dye molecule, Auramine O, were studied in the confined water environment in reversed micelles formed by ionic and nonionic surfactants. The fluorescence decays were measured by the fluorescence up-conversion method with a time resolution of <70 fs. The time-resolved fluorescence spectra were recreated and analysed using a one-dimensional generalised Smoluchowski equation assuming a time-dependent diffusion coefficient. The fluorescence decay times measured showed a dependence on water droplet sizes, and the reaction time was significantly slowed down in the smallest reversed micelles by both ionic AOT and nonionic surfactant. The reactive friction estimated from the Smoluchowski analysis was enhanced in the confined media which shows good agreement with the reaction times. Therefore, we found out that the interfacial charges are not required for the suppression of the reaction. Interestingly, the slower reaction dynamics were measured in nonionic surfactant reversed micelles than that in reversed micelles by AOT, even when Auramine O is in a similar size of water droplet.

Minako Kondo, Ismael A. Heisler, Stephen R. Meech

Chapter 12. Brownian Deposition of Nanodroplets and Nanofiber Growth via “Vapor–Liquid–Solid” Route

Simulation results of nanodroplet deposition on the wall of cylindrical reactor are presented. Additionally qualitative analytical estimations are given. Contributions of the Brownian diffusion of nanodroplets and thermophoresis are discussed. Application of deposited nanodroplets for the formation carbon nanofiber via “vapor–liquid–solid” route is briefly described.

Sergey P. Fisenko, Dmitry A. Takopulo

Chapter 13. Water Nanodroplets: Molecular Drag and Self-assembly

Directed transport and self-assembly of nanomaterials can potentially be facilitated by water nanodroplets, which could carry reactants or serve as a selective catalyst. We show by molecular dynamics simulations that water nanodroplets can be transported along and around the surfaces of vibrated carbon nanotubes. We show a second transport method where ions intercalated in carbon and boron-nitride nanotubes can be solvated at distance in polarizable nanodroplets adsorbed on their surfaces. When the ions are driven in the nanotubes by electric fields, the adsorbed droplets are dragged together with them. Finally, we demonstrate that water nanodroplets can activate and guide the folding of planar graphene nanostructures.

J. Russell, B. Wang, N. Patra, P. Král

Chapter 14. Atomistic Pseudopotential Theory of Droplet Epitaxial GaAs/AlGaAs Quantum Dots

In this chapter, following the introduction to the basic electronic properties of semiconductor quantum dots (QDs), we first briefly introduce our atomistic methodology for multi-million atom nanostructures, which is based on the empirical pseudopotential method for the solution of the single-particle

single-particle

problem combined with the configuration interaction (CI)

configuration interaction

scheme for the many-body problem which were developed in the solid-state theory group at the National Renewable Energy Laboratory over the past two decades. This methodology, described in Sect. 14.2, can be used to provide quantitative predictions of the electronic and optical properties of colloidal nanostructures containing thousands of atoms as well as epitaxial nanostructures containing several millions of atoms. In Sect. 14.3, we show how the multi-exciton spectra of a droplet epitaxy

droplet epitaxy

QD encodes nontrivial structural information that can be uncovered by atomistic many-body pseudopotential calculations. In Sect. 14.4, we investigate the vertical electric field tuning of the fine-structure splitting (FSS) in several InGaAs and GaAs QDs using our atomistic methodology. We reveal the influence of the atomic-scale structure on the exciton FSS in QDs. Finally, a comprehensive and quantitative analysis of the different mechanisms leading to HH–LH mixing

HH-LH mixing

in QDs is presented in Sect. 14.5. The novel quantum transmissibility

quantum transmissibility

of HH–LH mixing mediated by intermediate states is discovered. The design rules for optimization of the HH–LH mixing in QDs are given in this section.

Jun-Wei Luo, Gabriel Bester, Alex Zunger

Chapter 15. Local Droplet Etching: Self-assembled Nanoholes for Quantum Dots and Nanopillars

We review the mechanism and recent applications of the self-organized patterning of semiconductor surfaces by local droplet etching (LDE). LDE is a nanofabrication technique that is applicable in situ during molecular beam epitaxy (MBE) and fully compatible with state-of-the-art MBE systems. Most importantly, as a local etching technique that works with a number of different materials, it adds a new degree of freedom to established self-assembling techniques. During LDE, metallic droplets drill nanoholes into a semiconductor surface with structural parameters adjustable over a wide range by the process conditions. In subsequent overgrowth steps the holes are filled for the formation of nanostructures like, e.g., quantum dots (QDs)

Quantum dots

or quantum pillars. In comparison to other QD systems, the LDE dots have the key advantages that they are strain-free, highly uniform, and that their size is precisely adjustable. In addition, vertically stacked quantum dot molecules have been realized. Crystalline nanopillars are created by a combination of in situ LDE with ex situ selective etching that are highly attractive for studies of ballistic phonon and electron transport, e.g., in the field of thermoelectrics.

Christian Heyn, David Sonnenberg, Wolfgang Hansen

Backmatter

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