Метод трехмерной триангуляции в задачах кластерного анализа

Представлен метод многомерного шкалирования на основе трехмерной триангуляции. Рассмотрена возможность качественного сохранения геометрической структуры множества объектов при отображении многомерного пространства в трехмерное. Приведены результаты применения метода для решения задачи кластеризации на примере периодической системы элементов Д.И. Менделеева. Экспериментально показано, что по критериям качества кластеризации предложенный метод более эффективен в сравнении с методами k-средних и нейронной сети Кохонена.Представлено метод багатовимірного шкалювання на основі тривимірної тріангуляції. Розглянуто можливість якісного збереження геометричної структури множини об’єктів при відображенні багатовимірного простору в тривимірне. Наведено результати застосування методу для вирішення задачі кластеризації на прикладі періодичної системи елементів Д.І. Менделєєва. Експериментально показано, що за критеріями якості кластеризації запропонований метод більш ефективний у порівнянні з методами k-середніх та нейронної мережі Кохонена.The method of multidimensional scaling on the basis of the 3-D triangulation is presented. The qualitative preservation possibility of geometrical structure of objects by multidimensional space mapping to three- dimensional space is considered. The results of application of the method for clustering problem of the Mendeleyev periodic table are presented. It is experimentally shown, that the presented method is more effective by criteria of clustering quality in comparison with the methods of k-averages and the Kohonen neural network

Near-infrared Emitting CulnSe2/CulnS2 Dot Core/Rod Shell Heteronanorods by Sequential Cation Exchange

The direct synthesis of heteronanocrystals (HNCs) combining different ternary semiconductors is challenging and has not yet been successful. Here, we report a sequential topotactic cation exchange (CE) pathway that yields CuInSe2/CuInS2 dot core/rod shell nanorods with near-infrared luminescence. In our approach, the Cu+ extraction rate is coupled to the In3+ incorporation rate by the use of a stoichiometric trioctylphosphine-InCl3 complex, which fulfills the roles of both In-source and Cu-extracting agent. In this way, Cu+ ions can be extracted by trioctylphosphine ligands only when the In–P bond is broken. This results in readily available In3+ ions at the same surface site from which the Cu+ is extracted, making the process a direct place exchange reaction and shifting the overall energy balance in favor of the CE. Consequently, controlled cation exchange can occur even in large and anisotropic heterostructured nanocrystals with preservation of the size, shape, and heterostructuring of the template NCs into the product NCs. The cation exchange is self-limited, stopping when the ternary core/shell CuInSe2/CuInS2 composition is reached. The method is very versatile, successfully yielding a variety of luminescent CuInX2 (X = S, Se, and Te) quantum dots, nanorods, and HNCs, by using Cd-chalcogenide NCs and HNCs as templates. The approach reported here thus opens up routes toward materials with unprecedented properties, which would otherwise remain inaccessible

Зимостойкость некоторых видов рода Sorbus L. в Донбассе

Изучены анатомические особенности годичных побегов Sorbus persica Hedl., S. domestica L. и S. aucuparia L. Определены толщина тканей и степень лигнификации ксилемы. Наиболее развиты покровные и механические ткани у S. aucuparia, годичные побеги полностью вызревшие. У S. persica наблюдается отставание в развитии тканей и степени одревеснения побегов по сравнению с S. aucuparia. Отмечена высокая зимостойкость S. domestica и S. aucuparia и более низкая – S. persica. К перспективным видам отнесены S. domestica и S. aucuparia. Вид – S. persica нецелесообразно вводить в ассортимент декоративных растений юго-востока Украины.The anatomical peculiarities of annual spears of Sorbus persica Hedl., S. domestica L. and S. aucuparia L. were studied. The tissue depth and xylem lignification rate were defined. The tissues of S. aucuparia are most developed and annual spears are fully mature for vegetative period. S. persica lags in terms of tissue development and spear lignification rate compared to S. aucuparia. S. domestica and S. aucuparia revealed winter resistance at high level, whereas S. persica demonstrated it at low level. S. domestica and S. aucuparia are treated as perspective, while S. persica is unreasonable for introduction to the assortment of decorative plants of south-east of Ukraine

Before there was light : Excited state dynamics in luminescent (nano)materials

In this thesis we examine two types of luminescent materials: colloidal semiconductor nanocrystals (also known as quantum dots), and crystals doped with lanthanide ions. These materials convert one color of light to another. By investigating the dynamics of the excited state, we gain new insights into the physical processes that precede or compete with the emission of light. Such insights are necessary to enhance the efficiency of luminescent materials. We study blinking in individual semiconductor nanocrystals, i.e. the seemingly random switching between an emissive and a dark (non-emissive) state under continued excitation. The switching events are due to processes of charge carrier trapping and detrapping in or nearby the quantum dot, which we show occur much more frequently than usually assumed. After charge carrier trapping the nanocrystal is dark because of quenching by Auger processes. We investigate how losses due to Auger processes can be reduced: they are less efficient in CdSe/CdS core–shell nanocrystals or CdSe/CdS dot-in-rods than in bare CdSe nanocrystals. Crystals doped with lanthanide ions find numerous applications as luminescent materials in lighting, bioimaging and lasers. The properties of the material often depend on energy transfer processes between the lanthanide ions. These can be necessary to achieve the desired color output, but they can also reduce the photoluminescence efficiency. The quantitative understanding of energy transfer processes is however limited. We present and apply models to understand energy transfer processes in detail from the analysis of photoluminescence decay curves. In particular, we analyze the energy transfer processes that can result in downconversion (i.e. photoluminescence with a photon-to-photon conversion efficiency higher than 100%), between Tm3+ ions in Gd2O2S or from Tb3+ to Yb3+ in YPO4. Furthermore, we demonstrate that the efficiency of energy transfer processes doped nanocrystals is influenced by the refractive index of the environment of the crystal: the lower the refractive index of the environment, the higher the efficiency of energy transfer

Resolving the ambiguity in the relation between Stokes shift and Huang-Rhys parameter

Electronic transitions in luminescent molecules or centers in crystals couple to vibrations. This results in broadening of absorption and emission bands, as well as in the occurence of a Stokes shift EStokes. In principle, one can derive from EStokes the Huang–Rhys parameter S, which describes the microscopic details of the vibrational coupling and can be related to the equilibrium position offset ΔQe between the ground state and excited state. The commonly used textbook relations EStokes = (2S − 1)ℏω and EStokes = 2Sℏω are only approximately valid. In this paper we investigate how EStokes is related to S, taking into account the effects of a finite temperature. We show that in different ranges of temperature, different approximate relations between EStokes and S are appropriate. Moreover, we demonstrate that the difference between the barycenters of absorption and emission bands can be used to determine S in an unambiguous way. The position of the barycenter is, contrary to the Stokes shift, unaffected by temperature

Resolving the ambiguity in the relation between Stokes shift and Huang-Rhys parameter

Electronic transitions in luminescent molecules or centers in crystals couple to vibrations. This results in broadening of absorption and emission bands, as well as in the occurence of a Stokes shift EStokes. In principle, one can derive from EStokes the Huang–Rhys parameter S, which describes the microscopic details of the vibrational coupling and can be related to the equilibrium position offset ΔQe between the ground state and excited state. The commonly used textbook relations EStokes = (2S − 1)ℏω and EStokes = 2Sℏω are only approximately valid. In this paper we investigate how EStokes is related to S, taking into account the effects of a finite temperature. We show that in different ranges of temperature, different approximate relations between EStokes and S are appropriate. Moreover, we demonstrate that the difference between the barycenters of absorption and emission bands can be used to determine S in an unambiguous way. The position of the barycenter is, contrary to the Stokes shift, unaffected by temperature

Optoelectronic Properties of Ternary I-III-VI2 Semiconductor Nanocrystals: Bright Prospects with Elusive Origins

Colloidal nanocrystals of ternary I–III–VI2 semiconductors are emerging as promising alternatives to Cd- and Pb-chalcogenide nanocrystals because of their inherently lower toxicity, while still offering widely tunable photoluminescence. These properties make them promising materials for a variety of applications. However, the realization of their full potential has been hindered by both their underdeveloped synthesis and the poor understanding of their optoelectronic properties, whose origins are still under intense debate. In this Perspective, we provide novel insights on the latter aspect by critically discussing the accumulated body of knowledge on I–III–VI2 nanocrystals. From our analysis, we conclude that the luminescence in these nanomaterials most likely originates from the radiative recombination of a delocalized conduction band electron with a hole localized at the group-I cation, which results in broad bandwidths, large Stokes shifts, and long exciton lifetimes. Finally, we highlight the remaining open questions and propose experiments to address them

Optoelectronic Properties of Ternary I-III-VI2 Semiconductor Nanocrystals: Bright Prospects with Elusive Origins

Colloidal nanocrystals of ternary I–III–VI2 semiconductors are emerging as promising alternatives to Cd- and Pb-chalcogenide nanocrystals because of their inherently lower toxicity, while still offering widely tunable photoluminescence. These properties make them promising materials for a variety of applications. However, the realization of their full potential has been hindered by both their underdeveloped synthesis and the poor understanding of their optoelectronic properties, whose origins are still under intense debate. In this Perspective, we provide novel insights on the latter aspect by critically discussing the accumulated body of knowledge on I–III–VI2 nanocrystals. From our analysis, we conclude that the luminescence in these nanomaterials most likely originates from the radiative recombination of a delocalized conduction band electron with a hole localized at the group-I cation, which results in broad bandwidths, large Stokes shifts, and long exciton lifetimes. Finally, we highlight the remaining open questions and propose experiments to address them