Nanocrystals are vacancy free

Nanocrystals are predicted to be essentially vacancy free; their small size precludes any significant vacancy concentration. This simple result has important consequences for all thermomechnaical properties and processes such as creep and precipitation, which are based on the presence and migration of vacancies in the lattice.



Dislocations in nanocrystals


Crystal surfaces and interfaces play significant role in determining properties of nanocrystals compared with microcrystals. Dislocations which is planar defects in the crystalline structure of a solid is the determinant of mechanical properties of a material.

The free energy of a dislocation is made up of a number of terms

1- the core energy within a radius of about three lattice planes from the dislocation core
2- the elastic strain energy outside the core and extending to the boundaries of the crystal
3- the free energy arising from the entropy contributions

in microcrystals the first and second terms increase the free energy and are by far the most dominant terms. Hence dislocations unlike vacancies do not exist in thermal equilibrium.


Kelsall, Nanoscale


Mechanical alloying technique
.........at this value of d (Hall-Petch) deformation will no longer proceed by the generatin and propagation of matrx dislocations; the minimum grain size will then have been attained. This competition between the deformation and recovery behaviour is often cited as the reason for the generation of a characteristic d min for a given material. Once an entirely nanocrystalline structure is formed, further deformation is accomplished by grain boundary sliding GBS and there is no further refinement of structural scale. GBS does, however, accommodate deformation energy; this is stored with in the grain boundaries themselves and also in the form of strains within the nanograins which arise because of grain boundary stresses. Grain boundary energies may as a result, be higher than expected, as much as 25% higher than the value for the grain boundary energy in a coarse-grained sample.

Kelsall, p 257



Qantitative changes in the parameters that describe a network or the shape of the cell can radically change the way the network behaves.

Quantitative Modelling of Biological Systems
http://www.nyas.org/ebriefreps/main.asp?intEBriefID=460




Microstructural and defect population change in electron beam irradiated Ge:SiO2 MCVD glasses in the conditions of refractive index change writing

Electron spin resonance (ESR) and ultra violet (UV) optical absorption spectra are measured in Ge:SiO2 glasses elaborated by modified chemical vapor deposition (MCVD) and irradiated by an electron beam of medium energy (10–50 keV). The doses (a few C/cm2) like the electron energy used, correspond to conditions for writing refractive index change for optical applications. These data yielded information on defect population changes. Raman spectroscopy was used for correlating these changes to microstructural changes. We point out a 5.1 eV absorption bleaching like surprisingly the one occurring under 5 eV laser irradiation correlated to an absorption increases around (4.4 and 5.8 eV).

Journal of Non-Crystalline Solids, Volume 351, Issues 14-15, 15 May 2005,


Shape and size give rise to descriptive terms applied to the typical appearance, or habit of crystals. Some habits are distinctive of certain minerals, although most minerals exhibit many differing habits (the development of a particular habit is determined by the details of the conditions during the mineral formation/crystal growth). Crystal habit may mislead the inexperienced as a mineral's internal crystal system can be hidden or disguised. Factors influencing a crystal's habit include: a combination of two or more crystal forms; trace impurities present during growth; crystal twinning and growth conditions (i.e., heat, pressure, space). Minerals belonging to the same crystal system do not necessarily exhibit the same habit. Some habits of a mineral are unique to its variety and locality: For example, while most sapphires form elongate barrel-shaped crystals, those found in Montana form stout tabular crystals. Ordinarily, the latter habit is seen only in ruby.

Wikipedia.com



Competing interactions and patterns in Nanoworld

An experimental model made from small magnets was demonstrated which were free to rotate on different lattices. The geometry of magnets and the model as whole has been adapted to represent pure dipolar systems. ..................The patterns arising in nanosystems caused by competing interactions are classified into four main groups:

(i) self-competing interactions;
(ii) competition between a short- and a long-range interaction;
(iii) competition between interactions on a similar length scale; and
(iv) competition between interactions and anisotropy. Each class is further divided into subclasses corresponding to the localized and delocalized particles. For each subclass, concrete sets of interactions, corresponding patterns and microscopic details of systems where they appear are presented.

Wiley Interscience, 2007 WILEY-VCH Verlag GmbH & Co


Stone Wales

The Stone Wales defect is a defect that occurs on carbon nanotubes and is thought to have important implications for nanotube's mechanical properties. The defects are thought to be responsible for nanoscale plasticity and the brittle-ductile transitions in carbon nanotubes.

One of the plausible processes for isomerization of the fullerene is the so-called Stone-Wales or "pyracylene" transformation, which is the 90° rotation of two carbon atoms with respect to the midpoint of the bond. The Stone-Wales transformation is also used to describe the structural changes of sp²-bonded carbon nanosystems. For example, it has been proposed that the coalescence process of fullerenes or carbon nanotubes may occur through a sequence of such a rearrangement. By the Stone-Wales transformation, four hexagons are changed into two pentagons and two heptagons. It is a kind of Stone-Wales defect.

Wikipedia




Quantum Dots

The research of microelectronic materials is driven by the need to tailor electronic and optical properties for specific component applications. It has been made possible to fabricate artificial dedicated materials for microelectronics, where the electronic structure is tailored by changing the local material composition and by confining th electrons in nanometer size foils or grains. Due to quantization of electron energies, these systems are often called quantum structures. If the electrons are confided by a potential barrier in all three directions, the nanocrystals are called quantum dots. This review of quantum dots begins with discussion of the physical principles and first experiments and concludes with the first expected commercial applications single electron pumps, biomolecule markers and QDs lasers.
In nanocrystals, the crystal size dependency of the energy and the spacing of discrete electron levels are so large that they can be observed experimentally and utilized in technological applications. QDs are often also called mesoscopic atoms or artificial atoms to indicate that the scale of electron states in QDs is larger than the lattice constant of a crystal. However, there is no rigorous lower limit to the size of a QD and therefore even macro-molecules and single impurity atoms in a crystal can be called QDs.
The quantization of electron energies in nanometer size crystals leads to dramatic changes in transport and optical properties. ……..the colour change of the fluorescence is governed by the electron in a potential box effect familiar from elementary text books of modern physics.

In bulk crystals, each electron band consists of a continuum of electron states. however, the energy spacing of electron states increases with decreasing QD size, and therefore the energy spectrum of an electron band approaches a set of discrete lines in nanocrystals.
In quantum physics, the electronic structure is often anlyzed in the terms of the density of electron states DOS. The prominent transformation from the continuum of states in a bulk crystal to the set of discrete electron levels in a QD formulates that the DOS is proportional to the square root of the electron energy.

The handbook of nanotechnology; Nanometer Structures; p 111






Dislocation

dislocation is a crystallographic defect, or irregularity, within a crystal structure. The presence of dislocations strongly influences many of the properties of materials. Some types of dislocations can be visualised as being caused by the termination of a plane of atoms in the middle of a crystal. In such a case, the surrounding planes are not straight, but instead bend around the edge of the terminating plane so that the crystal structure is perfectly ordered on either side. The analogy with a stack of paper is apt: if a half a piece of paper is inserted in a stack of paper, the defect in the stack is only noticeable at the edge of the half sheet. wikipedia





Defects in crystalline

In research concerning metal particles, the relationship between the decagonal symmetry in the diffraction pattern and multiple-twin texture has been discussed extensively. Because such texture has been proven to be stable only when the size of the substance is less than about 20-30 run and disadvantageous energetically as the size of the substance increases due to elastic strain, the model is not applicable to the present solvates, which have a size of about 100 \un without any revision. From the standpoint of the multiple-twinning model, an experimental value (-35.5°)deviates less than 1° from the theoretical value (i.e. 2rc/10 = 36"). This gap may be compensated by the presence of some kinds of crystal defects. Thus, high-resolution transmission electron microscopy (HREM) was performed as an initial step in order to identify the nature of the defects in C70 C6 H5 CH3 The discrepancy is explained by the strain relaxation of the defects.

......According to dislocation theory, the strain energy of dislocation is thought to be proportional to the square of the displacement vector.

High-resolution electron microscopy of crystal defects in C70-toluene solvate, Oxfordjournals.org



The method selected, which favours the penetration of nanoparticles into the plants, involved injecting the bioferrofluid inside the internal hollow of the leaf petiole (Fig. 1B). In this way, the bioferrofluid penetrates into the plant and is translocated to other areas through the vascular tissues. Small magnets were placed on the petiole of the leaf opposite to the injection point and on some of the roots.

Nanoparticles as Smart Treatment-delivery Systems in Plants: Assessment of Different Techniques of Microscopy for their Visualization in Plant Tissues; Annals of Botany;Oxfordjournals 2008





The energy that is necessary to remove a Ga atom from a Ga site is called the VGa vacancy formation energy. Then an AsGa antisite defect (As atom) at another Ga site changes its position to the VGa vacancy site. To change the sites, an AsGa aitisite defect needs to overcome the potential barrier between the two sites. The potential barrier is called as the migration energy.

Annealing Induced Diffusion Dynamics in As Ion-Implanted GaAs; oxford journals, 2007


Structure of self-interstitial in some common metals. The left-hand side of each crystal type shows the perfect crystal and the right-hand side the one with a defect.






Interstitials are a variety of crystallographic defects, i.e. atoms which occupy a site in the crystal structure at which there is usually not an atom, or two or more atoms sharing one or more lattice sites such that the number of atoms is larger than the number of lattice sites. They are generally high energy configurations. Small atoms in some crystals can occupy interstitial sites in an energetically favourable configuration, such as hydrogen in palladium.

wikipedia






Lattice Defects in and Carious Dissolution of Human Enamel Crystals

Human enamel crystals under carious dissolution were studied by a combination of high resolution electron microscopy and microbeam electron diffraction. The crystals selected for study were those oriented in respect of the electron beam such that the electron-lucent small spots and larger areas due to carious dissolution could be examined in relation to the high resolution images of the (001) planes. Irregularities in the arrangement of the lattice fringes, including shifted, disrupted, and curved fringes, were observed in many cases of these electron-lucent spots and large areas. The disrupted lattice fringes were interpreted as being a type of edge dislocation. These results indicate that carious dissolution will initially attack the portion of the crystal with lattice defects.

Journal of Electron Microscopy 36(6): 387-391 (1987), Oxford Journals.org



We succeeded in distinguishing between oxygen and silicon atoms on an oxygen-adsorbed Si(111)7 × 7 surface, and also distinguished between silicon and tin atoms on Si(111)7 × 7-Sn intermixed and Si(111) × -Sn mosaic-phase surfaces using non-contact atomic force microscopy (NCAFM) at room temperature. Atom species of individual atoms are specified from the number of each atom in NC-AFM images, the tip-sample distance
dependence of NC-AFM images and/or the surface distribution of each atom. Further, based on the NC-AFM method but using soft nanoindentation, we achieved two kinds of mechanical vertical manipulation of individual atoms: removal of a selected Si adatom and deposition of a Si atom into a selected Si adatom vacancy on the Si(111)7 × 7 surface at 78 K.


Atom-selective imaging and mechanical atom manipulation using the non-contact atomic force microscopic