Nanostructures

Is defined as an object that has at least one dimension in the range of 1 – 100 nm. In describing nanostructures it is necessary to differentiate between the numbers of nanoscale dimensions:

1) Nanoclusters are sturctures that are 1 to 100 nm in each spatial dimension. These structures are categorized as 0D nanostructures. Some examples of 0D nanostructures include: Qunatum dots, Nanoparticle arrays, Core-shell nanoparticles, Hollow cubes and nanoshperes. The 0D nanostructures are shown in following figure.

Figure 1 - Typical scanning electron microscope (SEM) and transmission electron microscope (TEM) image of different types of 0D NSMs, which is synthesized by several research groups. (A) Quantum dots, (B) nanoparticles arrays, (C) core–shell nanoparticles, (D) hollow cubes, and (E) nanospheres. Image courtesy of http://www.materialstoday.com   

2) Nanotubes and nanowires have a characteristic diameter between 1 and 100 nm and a length that could be much greater. These structures are categorized as 1D nanostructures. Some examples of 1D nanostructures are Nanowires, Nanorods, Nanotubes, Nanobelts, Nanoribbons, and hierarchical nanostructures.

Figure 2 - Typical SEM image of different types of 1D NSMs, which is synthesized by several research groups. (A) Nanowires, (B)nanorods, (C) nanotubes, (D) nanobelts, (E) nanoribbons, and (F) hierarchical nanostructures. Image courtesy of http://www.materialstoday.com

3) Nanotextured surfaces or thin films have thickness between 1 and 100 nm, while the other two dimensions are much greater. These structures are categorized as 2 D nanostructures.

Figure 3 - Typical SEM and TEM image of different kinds of 2D NSMs, which is synthesized by our and several research groups.(A) Junctions (continuous islands), (B) branched structures, (C) nanoplates, (D) nanosheets , (E) nanowalls, and (F) nanodisks. Image courtesy of http://www.materialstoday.com

4. Finally, bulky materials with all dimensions above 100 nm, but contain 0D, 1D and/or 2D nanostructures are termed 3D nanostructures

0-D nanostructures

 Some of the 0-D nanostructures that we’ve mentioned previously are: Quantum dots, Nanoparticle arrays, Core-shell nanoparticles, hollow cubes and nanospheres. Nanoparticles are defined as small objects that are sized between 1 and 100 nm and that behave as a whole unit with respect to the transport properties.

Nanoparticles are size-dependent. Thus, the properties of materials change as their size approaches the nanoscale and as the percentage of atoms at the surface of a material becomes significant. Interesting and unexpected properties of nanoparticles are therefore significantly due to the large surface area of the material, which contributes more significantly to the material properties that the small bulk.

Just to clarify the bulk materials (these are particles that are larger than one micrometer) contain insignificant percentage of atoms at the surface in relation of the number of atoms in the bulk of the material, and therefore do not exhibit size dependent changes in their physical properties.

Nanoparticles possess unexpected optical properties as they are small enough to confine their electrons and produce quantum effects. The nanoparticles where first used unknowingly by artists as far back as the 9^th century for generating Glitter effects on the surface of pots or in colors used for stained glass. The unique physical properties allowed much higher absorption of solar radiation in photovoltaic cells that are composed of nanoparticles that it in thin films of continuous sheets of the same material.

Other size dependent property changes include quantum confinement in semiconductor particles, surface plasmon resonance in some metal particles, and chemical reactivity that are utilized for image formation in photography field.

1-D Nanostructures

1-D structures include nanowires, quantum wires, nanorods and nanotubes. Nanowire is a nanostructure with a diameter of nanoscale dimensions. In other words nanowiresis defined as structure that has a thickness or diameter constrained to tens of nanometers or less and an unconstrained length. At these scales, quantum mechanical effects are significant-which lead to the coining of the term “quantum wires”.

Figure 4 - NIST "grows" semiconductor nanowires that emit ultraviolet light as part of a project to make prototype nano-lasers and other devices and the measurement tools needed to characterize them. Electron micrograph shows the gallium nitride wires growing on a silicon substrate (color added for contrast.) Image courtesy of: http://www.nist.gov/pml/div686/growing_052506.cfm

There are different types of nanowires including metallic (e.g. Ni, Pt, Au), semiconductiong(e.g. Si, InP, GaN), and insulating (SiO2, TiO2). Molecular nanowires are composed of repeating molecular units that can be either organic (e.g. DNA) or inorganic(e.g. Mo6S9-xlx). There are also new forms of nanowires that include core-shell superlatice nanowires.

Nanowires have quantum confined directions, while still leaving one unconfined direction for electrical conduction.

Quantum confinement can be observed once the diameter of a material is of the same magnitude as the de Broglie wavelength of the electron wave function.When materials are this small, their electronic and optical properties deviate substantially from those of bulk materials. For more information on quantum confinement effect visit Wikipedia: Potential well.

This allows nanowires to be used in applications where electrical conduction is required. Because of their unique density of electronic states, nanowires with very small diameter are expected to exhibit significantly different optical, electrical and magnetic properties from their bulk 3D crystalline counterparts.

Inorganic nanotubes are often composed of metal oxides, and exhibit various advantages such as:

•  easy synthetic access and high crystallinity,
•  good uniformity and dispersion,
•  predefined electrical conductivity,
•  good adhesion to a number of polymers and
•  high impact resistance. 

They are therefore promising candidates as fillers for polymer composites with enhanced thermal, mechanical, and electrical properties. Inorganic nanotubes are heavier than carbon nanotubes and not as strong under tensile stress, but they are particularly strong under compression, leading to potential applications in impact-resistant applications such as bulletproof vests.

Two representative examples of inorganic nanotubes include boron nitride nanotubes and silicon carbide nanotubes. Boron nitride nanotubes are semiconducting nanotubes with predictable electronic properties independent of diameter and number of layers, resistance to oxidation (suited to high temperature use) and a Young’s modulus of 1.22 TPa. Silicon carbide nanotubes are resistant to oxidation, suitable for use in harsh environments, and the surface silicon atoms comprise an exterior that can be easily functionalized.

2-D Nanostructures

2D Nanostructures include thin films, planar quantum wells and superlattices. One of the major groups in 2D structures is thin films-two dimensional films with a thickness that range between 1-100 nm. When films are very thin, their electronic and optical properties deviate substantially from those of bulk materials. As the material is miniaturized towards nanoscale, the confining dimension naturally decreases, but the characteristics are no longer averaged by bulk. So energy in nanomaterials is not continuous it’s discrete and is measured in quanta.

Confinement of the electrons in these systems significantly changes their interactions with electromagnetic radiation. Electrons that are confined in the direction perpendicular to the substrate affect the wave functions as well as the density of the states. Similarly phonons that are confined in the direction that is perpendicular to the substrate affect the thermal transport.

All transport phenomena in 2D structures are highly affected by defects, boundaries, and interfaces that might exist in or at the vicinity of the thin film.

3-D Nanostructures

3D structures include bulk nanocrystalline films and nanocomposites.

Nanostructured Bulk Materails

There are three main categories of nanostructured bulk materials: crystalline materials, polycrystalline materials, and amorphous materials.Crystalline materials are composed of atoms, molecules or ions arranged in an orderly repeating pattern. In some cases, the regular ordering can continue unbroken over a large scale, for example in diamond structure, in which each diamond is a single crystal.Polycrystalline materials are solid objects that are large enough to see and handle that are not composed of a single crystal, but instead are made of a large number of single crystals, known as crystallites, whose size can vary from a few nanometers to several meters. Almost all common metals, and many ceramics, are polycrystalline. Amorphous materials, or non-crystalline solids, are solids that lack the long-range order characteristic of a crystal. However, amorphous materials have some short-range order at the atomic length scale due to the nature of chemical bonding. Such solids include glass, plastics and gels.

Nanocomposites

A nanocomposite is a multiphase solid material where one of the phases has one, two or three dimensions of less than 100 nm. In the broadest sense, this definition is usually taken to mean a solid combination of a bulk matrix and at least one nano-dimensional phase with properties different from those of the matrix due to dissimilarities in structure and chemistry. The mechanical, electrical, thermal, optical, electrochemical, and/or catalytic properties of the nanocomposite differ markedly from those of the individual component materials.

Size limits for these effects have been proposed: <5 nm for catalytic activity, <20 nm for making a hard magnetic material soft, <50 nm for refractive index changes, and <100 nm for achieving supermagnetism, mechanical strengthening or restricting matrix dislocation movement.

Nancomposites can include combinations of a bulk organic materials with organic nano-materials, a bulk inorganic material with inorganic materials, or a mix of the two.

The large amount of reinforcement surface area means that a relatively small amount of nanoscale reinforcement can have an observable effect on the macroscale properties of the composite. For example, adding carbon nanotubes improves the electrical and thermal conductivity. Other kinds of nanoparticulates may result in enhanced optical properties, dielectric properties, heat resistance or mechanical properties such as stiffness, strength and resistance to wear and damage.