Nucleation of colloids and macromolecules: does the nucleation pathway matter?

A recent description of diffusion-limited nucleation based on fluctuating hydrodynamics that extends classical nucleation theory predicts a very non-classical two-step scenario whereby nucleation is most likely to occur in spatially-extended, low-amplitude density fluctuations. In this paper, it is shown how the formalism can be used to determine the maximum probability of observing \emph{any} proposed nucleation pathway, thus allowing one to address the question as to their relative likelihood, including of the newly proposed pathway compared to classical scenarios. Calculations are presented for the nucleation of high-concentration bubbles in a low-concentration solution of globular proteins and it is found that the relative probabilities (new theory compared to classical result) for reaching a critical nucleus containing $N_c$ molecules scales as $e^{-N_c/3}$ thus indicating that for all but the smallest nuclei, the classical scenario is extremely unlikely.Comment: 7 pages, 5 figure

Quantum box energies as a route to the ground state levels of self-assembled InAs pyramidal dots

A theoretical investigation of the ground state electronic structure of InAs/GaAs quantum confined structures is presented. Energy levels of cuboids and pyramidal shaped dots are calculated using a single-band, constant-confining-potential model that in former applications has proved to reproduce well both the predictions of very sophisticated treatments and several features of many experimental photoluminescence spectra. A connection rule between their ground state energies is found which allows the calculation of the energy levels of pyramidal dots using those of cuboids of suitably chosen dimensions, whose solution requires considerably less computational effort. The purpose of this work is to provide experimentalists with a versatile and simple method to analyze their spectra. As an example, this rule is then applied to successfully reproduce the position of the ground state transition peaks of some experimental photoluminescence spectra of self-assembled pyramidal dots. Furthermore the rule is used to predict the dimensions of a pyramidal dot, starting from the knowledge of the ground state transition energy and an estimate for the aspect ratio Q. © 2000 American Institute of Physics

Stability of strained heteroepitaxial systems in (1+1) dimensions

We present a simple analytical model for the determination of the stable phases of strained heteroepitaxial systems in (1+1) dimensions. In order for this model to be consistent with a subsequent dynamic treatment, all expressions are adjusted to an atomistic Lennard-Jones system. Good agreement is obtained when the total energy is assumed to consist of two contributions: the surface energy and the elastic energy. As a result, we determine the stable phases as a function of the main ``control parameters'' (binding energies, coverage and lattice mismatch). We find that there exists no set of parameters leading to an array of islands as a stable configuration. We however show that a slight modification of the model can lead to the formation of stable arrays of islands.Comment: 11 pages, 14 figures, submitted to Physical Review

Classification of Ge hut clusters in the arrays formed by molecular beam epitaxy at low temperatures on the Si(001) surface

Morphological investigations and classification of Ge hut clusters forming the arrays of quantum dots on the Si(001) surface at low temperatures in the process of the ultrahigh vacuum molecular beam epitaxy have been carried out using in situ scanning tunnelling microscopy. Two main species of Ge hut clusters composing the arrays - pyramidal and wedge-shaped ones - have been found to have different atomic structures. The inference is made that shape transitions between pyramids and wedges are impossible. The nucleation probabilities of pyramids and wedges equal 1/2 at the initial stage of the array formation. The wedges become the dominating species as the amount of the deposited germanium is increased. A fraction and a density of the pyramids in the arrays are rapidly decreased with the growth of Ge coverage. The derivative types of the clusters - obelisks (or truncated wedges) and accreted wedges - have been revealed and investigated for the first time, they have been found to start dominating at high Ge coverages. The obelisks originate from the wedges as a result of their height limitation and further growth of trapezoid facets. The apexes of the obelisks are formed by sets of the parallel (001) ridges. The uniformity of the cluster arrays have been evidenced to be controlled by the length of the wedge-like clusters. At low growth temperatures (360C) nucleation of new clusters is observed during the array growth at all values of Ge coverage except for a particular point at which the arrays are more uniform than at higher or lower coverages. At higher (530C) temperatures cluster nucleation has not been observed after the initial stage of the array formation.Comment: 27 pages, 11 figures. A brief review of literature is added. Final version accepted to Physics-Uspekhi. Typos are correcte

Effect of the lattice misfit on the equilibrium shape of strained islands in Volmer-Weber growth

We have studied the effect of the misfit on the equilibrium shape of three-dimensional pyramidal islands grown on a foreign substrate in the case of incomplete wetting (Volmer-Weber mode of growth). We have found that tensile islands have smaller aspect ratios compared with compressed islands owing to its better adhesion to the substrate. The average strains of consecutive layers decrease faster with thickness in compressed than in tensile islands. The strains decrease rapidly with thickness, with the consequence that above a certain height, the upper layers of the pyramid become practically unstrained and does not contribute to a further reduction in the upper base. As a result, the truncated pyramids are not expected to transform into full pyramids. Our results are in good agreement with experimental observations in different systems.Comment: 6 pages, 7 figures. Accepted version, minor change

The stability of a crystal with diamond structure for patchy particles with tetrahedral symmetry

The phase diagram of model anisotropic particles with four attractive patches in a tetrahedral arrangement has been computed at two different values for the range of the potential, with the aim of investigating the conditions under which a diamond crystal can be formed. We find that the diamond phase is never stable for our longer-ranged potential. At low temperatures and pressures, the fluid freezes into a body-centred-cubic solid that can be viewed as two interpenetrating diamond lattices with a weak interaction between the two sublattices. Upon compression, an orientationally ordered face-centred-cubic crystal becomes more stable than the body-centred-cubic crystal, and at higher temperatures a plastic face-centered-cubic phase is stabilized by the increased entropy due to orientational disorder. A similar phase diagram is found for the shorter-ranged potential, but at low temperatures and pressures, we also find a region over which the diamond phase is thermodynamically favored over the body-centred-cubic phase. The higher vibrational entropy of the diamond structure with respect to the body-centred-cubic solid explains why it is stable even though the enthalpy of the latter phase is lower. Some preliminary studies on the growth of the diamond structure starting from a crystal seed were performed. Even though the diamond phase is never thermodynamically stable for the longer-ranged model, direct coexistence simulations of the interface between the fluid and the body-centred-cubic crystal and between the fluid and the diamond crystal show that, at sufficiently low pressures, it is quite probable that in both cases the solid grows into a diamond crystal, albeit involving some defects. These results highlight the importance of kinetic effects in the formation of diamond crystals in systems of patchy particles.Comment: 15 pages, 13 figure

Homogeneous nucleation near a second phase transition and Ostwald's step rule

Homogeneous nucleation of the new phase of one transition near a second phase transition is considered. The system has two phase transitions, we study the nucleation of the new phase of one of these transitions under conditions such that we are near or at the second phase transition. The second transition is an Ising-like transition and lies within the coexistence region of the first transition. It effects the formation of the new phase in two ways. The first is by reducing the nucleation barrier to direct nucleation. The second is by the system undergoing the second transition and transforming to a state in which the barrier to nucleation is greatly reduced. The second way occurs when the barrier to undergoing the second phase transition is less than that of the first phase transition, and is in accordance with Ostwald's rule.Comment: 11 pages, 5 figure

Communication: Non-monotonic supersaturation dependence of the nucleus size of crystals with anisotropically interacting molecules.

We study the nucleation of model two-dimensional crystals formed from anisotropically interacting molecules using kinetic Monte Carlo simulations and the forward flux sampling algorithm. The growth probability P(n) of a cluster of n molecules is measured while the supersaturation s and interaction anisotropy of the molecules are varied, in order to gain insight into the nucleation mechanism. It is found that with increasing degree of interaction anisotropy the nucleus size (defined as the cluster size at which P(n) = 0.5) can increase with increasing s, with sharp jumps at certain s values. Analysis of the cluster shape reveals that nucleation in the system studied is of a non-standard form, in that it embodies elements of both the classical nucleation theory and the density functional theory frameworks

Učestalost tromboembolijskih događaja u sistemskom obliku vaskulitisa i lokaliziranom kožnom vaskulitisu

Objectives: Vasculitis is a rare disease characterized by inflammation and necrosis of blood vessels. Inflammationinduced thrombosis is a hallmark of several autoimmune diseases, such as systemic lupus erythematosus (SLE ), rheumatoid arthritis (RA ), Sjögren’s syndrome (SS), and systemic vasculitis. In this study, we aimed to investigate the frequency of thromboembolic (TE) events in the systemic form of vasculitis and cutaneous vasculitis and to determine the possible risk factors for developing thromboembolic events. Methods: In this single-centre retrospective study, the patients diagnosed with systemic vasculitis and cutaneous leukocytoclastic vasculitis were included. Medical records of the patients that were treated at the Division of Rheumatology, Clinical Immunology, and Allergology University Hospital Centre Osijek in the period of 30 months (from November 2016. to June 2019.) were analysed. Results: Out of a total of 46 patients who were included in the study, 30 were diagnosed with systemic vasculitis, and 16 with cutaneous vasculitis. Statistically significant differences in relation to gender were found between the two groups (systemic vasculitis vs cutaneous vasculitis: the female gender 76.67% vs 43.75%; p=0.026), but there was no difference between the groups in relation to the age of disease onset. Thromboembolic events were found to be more frequent in the systemic form of the disease (p=0.0321). In the systemic vasculitis group, TE events were found in patients who suffered from involvement of multiple organ systems and in younger patients. Conclusion: Our research found that patients with systemic vasculitis, who are younger, with high disease activity, and who suffered from the involvement of multiple organ system, have an increased risk for developing TE events. In those patients, special attention should be paid to searching for additional TE risk factors, such as genetics, for the purpose of preventing unwanted events and applying TE prophylaxis on time.Uvod: Vaskulitis je rijetka bolest karakterizirana upalom i nekrozom krvnih žila. Upalom inducirana tromboza smatra se svojstvom nekoliko autoimunosnih bolesti poput sistemskog eritemskog lupusa (SLE ), reumatoidnog artritisa (RA ), Sjögrenova sindroma (SS) te sistemskih vaskulitisa. U ovom istraživanju nastojali smo utvrditi učestalost tromboembolijskih (TE) događaja kod sistemskog oblika vaskulitisa i vaskulitisa ograničenog na kožu te procijeniti koji su mogući rizični čimbenici za razvoj tromboembolijskih (TE) događaja. Ispitanici i metode: U ovoj retrospektivnoj studiji sudjelovali su bolesnici s dijagnozom sistemskog vaskulitisa i vaskulitisa ograničenog na kožu liječenih u Zavodu za reumatologiju, kliničku imunologiju i alergologiju Kliničkoga bolničkog centra Osijek u razdoblju od 30 mjeseci (od studenog 2016. do lipnja 2019. godine). Klinički podatci prikupljeni su pretraživanjem medicinske dokumentacije. Rezultati: U studiju je uključeno ukupno 46 bolesnika, 30 s dijagnozom sistemskog vaskulitisa i 16 s vaskulitisom ograničenim na kožu. Među dvjema skupinama bilo je statistički značajne razlike po spolu (sistemski vaskulitis vs vaskulitis ograničen na kožu – ženski spol 76,67% vs 43,75%; p=0,026), no među grupama nije bilo razlike u dobi pojave bolesti. Tromboembolijski događaji bili su češći u bolesnika sa sistemskim oblikom vaskulitisa (p=0,0321). Pri analizi bolesnika sa sistemskim vaskulitisom kao rizični čimbenici za razvoj TE događaja utvrđeni su zahvaćanje više organskih sustava bolešću te mlađa životna dob. Zaključak: U našem istraživanju pokazali smo da bolesnici sa sistemskim vaskulitisom, mlađe životne dobi, uz visoku aktivnost bolesti i zahvaćanje više organskih sustava imaju povećan rizik za nastanak tromoebolijskih događaja te bi se kod takvih bolesnika trebala posvetiti posebna pozornost u otkrivanju dodatnih rizičnih čimbenika kao što je genetska predispozicija u svrhu sprječavanja neželjenih događaja te primjenu pravovremene TE profilakse

Coherent Stranski-Krastanov growth in 1+1 dimensions with anharmonic interactions: An equilibrium study

The formation of coherently strained three-dimensional islands on top of the wetting layer in Stranski-Krastanov mode of growth is considered in a model in 1+1 dimensions accounting for the anharmonicity and non-convexity of the real interatomic forces. It is shown that coherent 3D islands can be expected to form in compressed rather than in expanded overlayers beyond a critical lattice misfit. In the latter case the classical Stranski-Krastanov growth is expected to occur because the misfit dislocations can become energetically favored at smaller island sizes. The thermodynamic reason for coherent 3D islanding is the incomplete wetting owing to the weaker adhesion of the edge atoms. Monolayer height islands with a critical size appear as necessary precursors of the 3D islands. The latter explains the experimentally observed narrow size distribution of the 3D islands. The 2D-3D transformation takes place by consecutive rearrangements of mono- to bilayer, bi- to trilayer islands, etc., after exceeding the corresponding critical sizes. The rearrangements are initiated by nucleation events each next one requiring to overcome a lower energetic barrier. The model is in good qualitative agreement with available experimental observations.Comment: 12 pages text, 15 figures, Accepted in Phys.Rev.B, Vol.61, No2