21st Century Nanoscience – A Handbook

Exotic Nanostructures and Quantum Systems (Volume Five)

Klaus D. Sattler

• Publisher: CRC Press
• Year: 2020
• ISBN:
  9781000699876
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Nanotechnology & MEMS
• Index: Technology & Engineering

This 21st Century Nanoscience Handbook will be the most comprehensive, up-to-date large reference work for the field of nanoscience. Handbook of Nanophysics, by the same editor, published in the fall of 2010, embraced as the first comprehensive reference to consider both fundamental and applied aspects of nanophysics. This follow-up project has been conceived as a necessary expansion and full update that considers the significant advances made in the field since 2010. It goes well beyond the physics as warranted by recent developments in the field. The fifth volume in a ten-volume set covers exotic nanostructures and quantum systems.

Key Features:

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• Provides the most comprehensive, up-to-date large reference work for the field.

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• Chapters written by international experts in the field.

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• Emphasises presentation and real results and applications.

This handbook distinguishes itself from other works by its breadth of coverage, readability and timely topics. The intended readership is very broad, from students and instructors to engineers, physicists, chemists, biologists, biomedical researchers, industry professionals, governmental scientists, and others whose work is impacted by nanotechnology. It will be an indispensable resource in academic, government, and industry libraries worldwide. The fields impacted by nanoscience extend from materials science and engineering to biotechnology, biomedical engineering, medicine, electrical engineering, pharmaceutical science, computer technology, aerospace engineering, mechanical engineering, food science, and beyond.

1

Novel Nanoscience in Superfluid Helium

Arin Mizouri, Charlotte Pughe,
Berlian Sitorus, Andrew M.
Ellis, and Shengfu Yang

University of Leicester

Berlian Sitorus

Tanjungpura University

1.1 The Properties of Superfluid Helium

Viscosity

Thermal Conductivity

Quantized Vortices

1.2 Growth of Nanomaterials in Bulk Superfluid Helium

Mechanism for Nanowire Formation

Typical Bulk Superfluid Helium Apparatus

Nanostructures Formed in Bulk Superfluid Helium

1.3 Synthesis of Nanomaterials in Superfluid Helium Droplets

Basics of Helium Droplets

Formation of Nanoparticles in Helium Droplets

Quantized Vortices and the Growth of Nanowires Using Superfluid Helium Droplets

1.4 Perspectives and Challenges

References

Nanoscience and nanotechnology are broad interdisciplinary areas of research that span chemistry, physics, materials science and biology. The motivation for studying these areas is driven partly by the fascinating properties offered by new materials at the nanoscale, which can differ drastically from their bulk counterparts, and can be exploited for applications in medicine,^1,2 electronics and opto-electronics,^3,4 catalysis,^5–7 and nowadays in consumer products such as clothing and cosmetics.^8,9 The properties of nanomaterials are strongly dependent on their shapes, sizes, morphology, structures and chemical composition, and it is critical to have control over these parameters in order to obtain novel materials with desired properties.

To date, the vast majority of nanomaterials have been synthesized at the room temperature or above, but low-temperature techniques have also emerged in recent years, e.g., by employing solid matrices or superfluid helium as the solvents. In the solid matrix method, hot atoms collide with a cold solid matrix formed by the condensation of rare gases, nitrogen or carbon dioxide,^10–12 which then melts and allows atoms to aggregate. This is the first step occurring in the formation of nanomaterials in the solid matrix. While the clusters are being formed, the binding energy between atoms is released, stimulating further melting and thereby allowing subsequent migration and clustering of the added materials. In contrast, the formation of nanomaterials in superfluid helium occurs in the liquid phase, where the migration and aggregation is easier than in solid matrices. The extraordinary properties of superfluid helium, such as near-zero viscosity, ultrahigh thermal conductivity and quantized vortices,¹³ allow the creation of a variety of nanostructures, many being inaccessible by conventional synthetic methods. However, super-fluid helium is intrinsically a poor solvent as helium atoms interact very weakly with other atoms and molecules. As a result, materials added tend to locate at the walls of the container, rather than reside within the fluid, in bulk super-fluid helium.

Problems associated with the condensation of samples on the container walls can be circumvented by using liquid helium droplets, which are isolated, self-contained large clusters of helium typically composed of 10³–10¹¹ helium atoms.^14,15 Unlike bulk liquid helium, helium droplets have no container wall; so foreign species can be trapped and then confined by the surface of the droplet. Toennies et al. were the first to demonstrate that the superfluidity of bulk liquid helium also occurs on the nanoscale,^16,17 to which atoms/molecules can be added by a pickup technique,¹⁸ i.e., by collisions between the droplets and gas-phase atoms/molecules. Helium droplets are highly “sticky”, with a near unity pickup probability. In other words, the number of atoms/molecules being added to the droplets is approximately the number of collision events. By controlling the pickup conditions, formation of nanoparticles containing a few dozens to millions of atoms are possible, which depends on the size of the droplets and the number density of atoms/molecules in the pickup region. Although still in its infancy, a number of metallic nanoparticles have been synthesised using this technique. In 2011, Loginov et al. performed the first transmission electron microscopy (TEM) imaging of spherical metallic nanoparticles formed in helium droplets.¹⁹

One of the unique advantages of using helium droplets as nano–reactors to grow nanomaterials is the ease of forming core–shell structures, which can be achieved simply by pickup of different types of materials when they pass through two sequential pickup regions. Such a scenario was first demonstrated by the formation of molecular clusters, where a core–shell structure was deduced using mass spectrometry.²⁰ Evidence for the formation of a core–shell bimetallic structure was obtained in Ni/Au nanoparticles formed by sequential addition of Ni and Au to helium droplets, for which the core–shell structure was evident from X-ray photoelectron spectroscopy (XPS).²¹ In this process the inter-diffusion between layers is also minimized as the reactions occur in very cold superfluid helium, giving rise to core–shell structures with clear boundary at the interface, which is difficult to achieve using conventional “hot” approaches.

This chapter is divided into three sections: (i) the properties of superfluid helium (in bulk helium and droplets), and how and why each property is useful in the synthesis of nanomaterials; (ii) fabrication of nanostructures in bulk superfluid helium; and (iii) the new possibilities for the synthesis of nanomaterials using superfluid helium droplets, and the recent advances in this emerging field of nanoscience. Finally, we will close by considering some of the remaining challenges and the prospects for superfluid helium being employed in the development of next-generation materials.

1.1 The Properties of Superfluid Helium

Helium is the only element that remains as a liquid at 0 K. This is a consequence of the very weak van der Waals interaction between helium atoms (5 cm^–1) and the low atomic mass (4 amu). This results in a zero-point vibrational energy being comparable to the interatomic attraction, preventing helium atoms from being pinned to a solid lattice unless a very high pressure is imposed. This very weak interaction also means a very low temperature is needed to liquefy helium. Liquid helium was first discovered in 1908 by Kammerlingh-Onnes, who cooled the element to 4.2 K. At this temperature, helium behaves as a normal liquid (He I phase)²² but upon further cooling below 2.17 K it enters a superfluid phase (He II, see Figure 1.1 for the phase diagram of ⁴He). Superfluidity is the consequence of Bose-Einstein condensation (BEC), in which all of the bosonic particles occupy the same quantum state.²³ In contrast, ³He is a fermion and has a distinctly different phase diagram when compared with ⁴He. Although ³He can become a super-fluid, the transition temperature is far lower (3 mK)²⁴ in order for the fermionic ³He atoms to pair up and form quasi-bosons.

FIGURE 1.1 Phase diagram of helium. The three dots at 2.17 K, 1.76 and 4.2 K highlight the -point, upper -point and boiling point of helium, respectively.

One of the major differences between bulk helium and helium droplets is that the temperature of bulk liquid helium can be varied via cooling or heating its container. In contrast, helium droplets have a steady-state temperature that is determined by the balance of the surface tension of the droplets and the kinetic energy carried by helium atoms. For ⁴He, this steady-state temperature is 0.37 K, which is well below the “kick-in” temperature for forming superfluid helium.²⁵

1.1.1 Viscosity

Kapitza, Allen and Misener performed the first experimental measurements showing the superfluidity of liquid helium below 2.17 K. In these experiments, performed in 1938, they investigated the viscous flow of helium through narrow channels.^26,27 Above 2.17 K, the liquid helium was found to be viscous, just like any normal liquids but below the -point the viscosity was found to decrease by a factor of 10⁶, allowing liquid helium to flow through microchannels at driven by a tiny pressure difference. From this observation, Kapitza coined the name “superfluid” to describe frictionless flow in the He II phase.

Some of the unusual properties of bulk superfluid helium irrelevant to the growth of nanomaterials, such as the fountain effect, film flow and creeping, will not be covered here. Instead, we will focus only on properties that are possessed by both bulk superfluid helium and helium droplets and that are important for the growth of nanomaterials, such as superfluidity (which allows the free motion of dopants and thus easy aggregation), the ultrahigh cooling rate (allowing different species to be incorporated) and quantized vortices (allowing the formation of 1-D nanostructures).

Not only does the superfluid flow unhindered, but also species contained within the fluid are able to move freely, almost unperturbed by the surrounding helium atoms. Due to the poor solubility of dopants in liquid helium, frictionless motion of impurities makes it difficult to suspend a species inside the bulk superfluid. This difficulty can be circumvented by using helium droplets. In 1998, Toennies et al. showed for the first time that atoms and molecules can be confined in very small droplets of liquid helium,¹⁶ which can be retained for further investigations, e.g., by mass spectrometry and/or optical spectroscopy. The first experimental evidence for the superfluidity of helium droplets was obtained by Grebenev et al., who measured the rotationally resolved infrared spectrum of the OCS molecule.²⁸ The spectrum obtained showed well-resolved P and R branches (see Figure 1.2), suggesting that the OCS molecule is able to rotate in a way similar to gas phase mole...

Table of contents

• Cover
• Half Title
• Series Page
• Title Page
• Copyright Page
• Table of Contents
• Editor
• Contributors
• 1 Novel Nanoscience in Superfluid Helium
• 2 Stimuli-Responsive Polymeric Nanomaterials
• 3 Nanoparticle Superlattices
• 4 Heptacene
• 5 Epitaxial Silicene
• 6 Emissive Nanomaterials and Liquid Crystals
• 7 Nanoscale Alloys and Intermetallics: Recent Progresses in Catalysis
• 8 Nanoionics: Fundamentals and Applications
• 9 Structure-Dynamic Approach of Nanoionics
• 10 Energetic Processing of Molecular and Metallic Nanoparticles by Ion Impact
• 11 Nanoscale Fluid Dynamics
• 12 Transport in Nanoporous Materials
• 13 Beyond Phenomena: Functionalization of Nanofluidics Based on Nano-in-Nano Integration Technology
• 14 Classical Density Functional Theory and Nanofluidics: Adsorption and the Interface Binding Potential
• 15 Water Flow in Graphene Nanochannels
• 16 Transport of Water in Graphene Nanochannels
• 17 Nanoscale Magnetism
• 18 Physics of Nanomagnets
• 19 Magnetic Disorder at the Nanoscale
• 20 The Study of Hexagonal Fe2Si: In Terms of Its Structure and Electronic Properties
• 21 Tunable Picosecond Magnetization Dynamics in Ferromagnetic Nanostructures
• 22 Nanothermodynamics: Fundamentals and Applications
• 23 Characterization of Nanoscale Thermal Conductivity
• 24 Nanothermometers: Remote Sensors for Temperature Mapping at the Nanoscale
• 25 Luminescence Nanothermometry
• 26 Diamond Nanothermometry
• Index