Define the yield stress. 1 What is the band structure of solids? 2 What is a nanocrystal? 3 Give three examples of applications of nanofibers. 3 Describe the fabrication method called self-organizations, and tell which materials can be obtained that way. 4 What is the carbon nanotube? 4 What is the crystal structure of a diamond. 5 Define the coefficient of variation and explain why this is necessary to be used to describe the properties of nanometals? 5 How would you define nanotechnology? 7 What is the characteristic size of semiconductor quantum dot? 7 What does it mean energy gap in the band structure theory of solids 7 What is the necessary condition to be fulfilled to create a quantum well of two semiconductor materials? 7 The energy of electron in a quantum well with infinite barriers is 25 meV. What would be this energy if the quantum well width is decreased twice? 8 What does it mean that the light is linearly polarized? 8 Explain shortly what are the electrostatically formed quantum dots? 9 What can You tell about the term wave function? 9 What is the crystallographic structure of graphite? 10 How to obtain/fabricate graphene? 10 List, please, at least three characteristic physical properties of graphene 10 List, please, at least three methods of fabrication of carbon nanotubes 11 Define shortly the group of materials called nanofibers? 11 Define shortly nanofiber fabrication technique called drawing the nanofibers? 11 Give at least three reasons making the electrospinning a large scale production (industrial) techniques of fabricating nanofibers? 11 What is the dependence of the specific surface area on the grain size in e.g. nanometals? 12 What is the dependence of yield stress on the grain size of nanometal 12 Give the three examples of fabrication techniques of nanometals. 13 Select on application of nanometals and explain it more in detail, underlining the physical property of a nanometal which is explored in this given application. 13

1. Define the yield stress.Yield stress: strain above which a plastic irreversible deformation occurs (defined for 0,002 of the relative deformation). Determined typically during tensile test.Hall-Petch equation:

Where: limiting strain; Yield stress, constant of strain character, proportionality constant, grain size.

The inversed Halla-Petcha relationship with negative k value (appearance of other deformation accommodation mechanisms, e.g. via slippage over the grain boundaries) means reaching the optimal mean size of the grains for which the material exhibits maximal mechanical parameter.Theres a big influence of grain size on the yield stress. Hardiness increase with the grain size decrease (Hall-Petch relationship). The yield stress increases for smaller values of CV (Coefficient of variation) - more uniform distribution.

2. What is the band structure of solids?

The energy range of electrons in solid (conductor). Electronic band structure (or simply band structure) of a solid describes those ranges of energy that an electron within the solid may have (called energy bands, allowed bands, or simply bands), and ranges of energy that it may not have (called band gaps or forbidden bands)

Energy band structures electron in a crystal:

3. What is a nanocrystal?

Nanocrystalites (usually made of semiconductors, e.g. Si, CdSe) aggregated after reactions from a chemical solution. - many fabrication techniques, many materials used, and many forms of nanocrystals.Main advantage: many can be easily solved in water bio-medical applications (labeling the disease affected areas, or drag transportation)Fundamental properties: crystallite (of semiconductor) in insulating matrix (often amorphic or liquid) strong carrier confinement in 3D cheap technology and many applications (mainly in biology and medicine, but also (opto)electronics) strong surface effectsA nanocrystal is a crystalline nanoparticle. Some sources define it as any singlecrystalline nanomaterial with at least one dimension 100 nm., while others define it as a nanoparticle with any kind of crystalline structure including e.g., multiply twinned configurations.A material object that measures less than 1 micrometer, i.e., 1000 nanometers in all dimensions is a nanoparticle, not a nanocrystal. Only single-crystalline or polycrystalline materials are nanocrystals.

4. Give three examples of applications of nanofibers.

Medicine: artificial organs, tissue engineering, implants, scaffolds for bone tissues, drug transportation, medical materials (medical dressings) Protective materials: materials absorbing the acoustic wave, protective textiles (military), sensing materials Textiles: sport equipment, shoes, waterproof wardrobe, hygienic textiles (e.g. textiles containing active substances like antiseptic or dye chemical sensors) Filtration: vacuum, air, gasses, oil, fuel, water, beverages, etc. Dyes in cosmetics Storage and transfer of energy: solar cells, batteries, membrane fuel cells Chemical carriers (in catalysis and air/water purification) In civil engineering: insulating materials, modifying materials (in epoxy resins), dye carriers in glues and adhesives

5. Describe the fabrication method called self-organizations, and tell which materials can be obtained that way.

Molecular self-assembly self-forming of spatially organized structures in a form of long fibers. Mechanism based on the forces between the particles leading to the formation (precipitation) of fiber-like structures. The final shape of the nanofiber depends on the shape of the base particles. Applic. Of method : Lab. Scalability : no Easy implementation : no Control of nanofiber sizes : no Fiber length : 1 20 mAdvantages: Small fiber sizes Nets of fibers Control of mechanical propertiesDisadvantages: ComplexityMaterials that can be obtained that way: Nanofibers Epitaxial QDs - easy process of self-formation and partial self-organization on a surface.

6. What is the carbon nanotube?

Carbon nanotube - Rolled-up layer of graphen. Properties depend on: Direction of rolling-up Size (diameter) of nanotube

Methods of fabrication: Burning process (e.g. hydrocarbons) Electric arc method Laser ablation Gas phase deposition

7. What is the crystal structure of a diamond.

In diamond crystal each carbon atom is surrounded by 4 other carbon atoms tetrahedron structure (covalent bonds of sp3 hybridization). Such tetrahedrons form the diamond crystal surface centered cubic (regular) structure. Diamond properties (chemical and mechanical stability) originate from very strong covalent bonds between the atoms in the crystal and high spatial density of atoms I the crystallographic structure.

8. Define the coefficient of variation and explain why this is necessary to be used to describe the properties of nanometals?

Coefficient of variation (figure of merit of the distribution quality) in function of grain size for metals obtained by hydrostatic pressing method. CV increases with decreasing grain size. The yield stress increases for smaller values of CV (more uniform distribution). CV increases with decreasing grain size

- coefficient of variation, - standard deviation, - mean value.The yield stress increases for smaller values of CV (more uniform distribution)

How would you define nanotechnology?

Nanotechnology (sometimes shortened to "nanotech") is the manipulation of matter on an atomic, molecular, and supramolecular scale. The earliest, widespread description of nanotechnology referred to the particular technological goal of precisely manipulating atoms and molecules for fabrication of macroscale products, also now referred to as molecular nanotechnology. A more generalized description of nanotechnology was subsequently established by the National Nanotechnology Initiative, which defines nanotechnology as the manipulation of matter with at least one dimension sized from 1 to 100 nanometers.

What is the characteristic size of semiconductor quantum dot?

Size scale of semiconductor quantum dot is : 10 nm.

What does it mean energy gap in the band structure theory of solids?

The energy range of electrons in solid in which theres strong scattering of electrons on the atoms. As a result electrons of this energy range are not in the system.

What is the necessary condition to be fulfilled to create a quantum well of two semiconductor materials?The effects of quantum confinement take place when the quantum well thickness becomes comparable to the de Broglie wavelength of the carriers (generally electrons and holes), leading to energy levels called "energy subbands", i.e., the carriers can only have discrete energy value. Quantum wells are formed in semiconductors by having a material, like gallium arsenide sandwiched between two layers of a material with a wider bandgap, like aluminium arsenide.

The energy of electron in a quantum well with infinite barriers is 25 meV. What would be this energy if the quantum well width is decreased twice?

With increasing L (potential well width, increasing size of the quantum box also in 3D) the energy of the confined electron states decreases.

What does it mean that the light is linearly polarized?

In electrodynamics, linear polarization or plane polarization of electromagnetic radiation is a confinement of the electric field vector or magnetic field vector to a given plane along the direction of propagation. The orientation of a linearly polarized electromagnetic wave is defined by the direction of the electric field vector. For example, if the electric field vector is vertical (alternately up and down as the wave travels) the radiation is said to be vertically polarized.Light in the form of a plane wave in space is said to be linearly polarized. Light is a transverse electromagnetic wave, but natural light is generally unpolarized, all planes of propagation being equally probable. If light is composed of two plane waves of equal amplitude by differing in phase by 90, then the light is said to be circularly polarized. If two plane waves of differing amplitude are related in phase by 90, or if the relative phase is other than 90 then the light is said to be elliptically polarized.

Explain shortly what are the electrostatically formed quantum dots?

In electrostatic (gated) quantum dots, the potential confining the electrons is generated by the electrostatic field, which is created by the external voltages applied to the leads. Changing the geometry of the nanodevice we can obtain a diverse class of confinement potentials

What can You tell about the term wave function?

A wave function in quantum mechanics describes the quantum state of a particle and how it behaves. Typically, its values are complex numbers and, for a single particle, it is a function of space and time. The Schrdinger equation describes how the wave function evolves over time. The most common symbols for a wave function are or .State of a particle (e.g. electron) is described by its wave function complex in general:

Usually, the spatial and time variables can be separated:

Describes the wave of probability, where the probability density means probability if finding the particle in a given space In order to find the wave function (and probability density) it is necessary to know its energy and solve the Schrdinger equation:

What is the crystallographic structure of graphite?

In plane strong covalent bonds of sp2 hybridization in a hexagonal structure.Van der Waals bonding between the atomic layers layered structure of the entire crystal.

How to obtain/fabricate graphene?

Two main techniques:Exfoliation separating single crystallic layers from graphiteEpitaxy deposition of single carbon layer on a substrate

List, please, at least three characteristic physical properties of graphene. Massless electrons travel in graphene crystal with relativistic velocities approx. 1/300 of light speed Large mobility of charge carriers ( 104 cm2/Vs at room temp.) and high carrier concentration (n 1013 cm2) very good conductivity (low resistance) - j > 108 A/cm2 High thermal conductivity Almost fully transparent for visible light (absorbs only 2.3% of the intensity) It is over 100 times mechanically stronger than steel, and simultaneously elastic (can be expanded by 20% without plastic destruction) A membrane of oxidized graphene presents selective permeability does not transmits most of the gases including helium, but can well transmit water

List, please, at least three methods of fabrication of carbon nanotubes. Burning process (e.g. hydrocarbons) nanotubes can be detected in carbon black (spontaneous and uncontrolled process without application prospects) Electric arc method nanotubes were observed (accidently) in carbon black of graphite electrodes (for the first time in 1991) Then the method used for mass production and industrial fabrication (synthesis) of long carbon nanotubes (> 50 m) Laser ablation evaporation of carbon atoms from graphite under high intensity laser pulse (used from 1995). Method more expensive than the electric arc, but offering better efficiency and control of nanotubes properties Gas phase deposition nanotubes of even cm in length. Fabrication of regular matrices (also vertical). Prospects for mass production (the method yield by approx. 100 better than laser ablation).

Define shortly the group of materials called nanofibers?Materials of fibrous structure, the diameter in the range of approx. 50-500 nm and the length at least 100 times larger.

Define shortly nanofiber fabrication technique called drawing the nanofibers? Can be employed for e.g. sodium citrate (organic compund; used as gustatory and preserving substance added into food - E331; formula - C6H5Na3O7 ) Solution in chloroauric acid Drawing chains (fibers) by a micropipette (controlled by a mechanical microcontroller) from a droplet of the solution Rate 0.1 mm/s (i.e. 1 m per 3 h.) Then depositing the fiber on a substrate

Give at least three reasons making the electrospinning a large scale production (industrial) techniques of fabricating nanofibers?

Electrospinning fabrication of nanofibers from a polymer melt or their solutions by using electric field Scalability Easy implementation Control of nanofiber sizes Low costs Large length of fibers Fiber diameter : 3 1000 nm

What is the dependence of the specific surface area on the grain size in e.g. nanometals?

Nanometals polycrystaline materials made of common metal atoms (metallic chemical elements) with crystalites smaller than 1 m. similarly as in conventional metals these are polycrystalline materials but the grain size is in nm range (at least in one direction of below 100 nm) Drastic increase of the specific surface area (surface area divided by the volume)Scaling the specific surface area:

What is the dependence of yield stress on the grain size of nanometal?

Hardiness increase with the grain size decrease (Hall-Petch relationship).Hall-Petch equation:

Where: limiting strain; Yield stress, constant of strain character, proportionality constant, grain size.

Give the three examples of fabrication techniques of nanometals.

Both bottom up and top down approaches are used: gas phase and liquid phase epitaxy (deposition) Rapid cooling and nanocrystalization from amorphous phase Consolidation of nanopowders Method of cumulative (huge) plastic deformation

Select on application of nanometals and explain it more in detail, underlining the physical property of a nanometal which is explored in this given application. Silver nanoparticles as an antiseptic substance in materials for civil engineering : paints, lacquers, cement, concrete, etc. (usually in a colloidal form)Silver kills many of bacteries and fungus, and hence the materials remain clear and keep the original colours. silver is friendly to the environment, non-toxic and safe for human. Nanometals in transport industryThe main motivation is decreasing the mass of the vehicles (which implies decreased emission of CO2) via using materials with improved hardiness and smaller density. Application of nanometals is prospective from that point of view. It can also reduce the consumption of material during fabrication process.

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