Low dimensional structures & devices: Part 1

A crystal is defined as a solid material that is inclusive of the molecules, atoms, and ions that are arranged in an orderly fashion and a repetitive pattern in a spatial arrangement. Crystal growth is a major step in the process of crystallization. The process of crystal growth and kinetics is a dynamic process that is impacted by several factors. Crystals of a compound are a form of solid state that is one the purest form of the solid. The crystals are produced through the process of crystallization that is inclusive of two primary processes. The first process is nucleation, and the second process is crystal growth. In low supersaturation or desupersaturation, crystals are grown and accelerated with nucleation. This results in larger crystal size and distribution. A supersaturated solution is present in thermodynamic equilibrium with the solid phase at a specified temperature. The state of supersaturation is critical for the operation of the crystallization experiment. The crystallization process in the pharmaceutical sciences also helps in improved downstream processes that include filtration, milling and drying. These applications demand a crystallization of even more complicated molecules that can be used as drugs and duplication at a large scale. This paper discusses the process of crystallization of paracetamol in the solvent, isopropanol. The developed crystals have been studied in this thesis by application of the process supersaturation. The developed crystals have also been analyzed by using the PAT tools. The bioavailability of the compounds is essentially dependent on three factors that include solubility, permeability, and dosage. Based on the solubility of the compounds, the physiological pH value and permeability parameters of drugs are classified into several categories. These properties are altered and are impacted by the process of crystallization and crystal growth in the chemical setting. Paracetamol has three polymorphs with the form I being most stable and form II being the least stable. The process of crystal growth and crystallization focuses on the inclusion of the development of the most stable crystal polymorph to be used as a drug and to be mass-produced and applied in the pharmaceutical industry. The crystal data of the molecule reveals that the crystal growth of the molecule is subject to change with alterations in temperature. The bond length and bond angles for the crystal polymorphs of the paracetamol remain unchanged. The optimized geometry for a free molecule crystal is planar, where the phenyl rings and the acetamide lie in different planes. Bulk crystallization of paracetamol has been done as per the controlled conditions in clinical conditions for saturation experiments. The induction and the nucleation time in the process are determined by the changes in the concentration of the solute in the process of crystal growth. The products of the process are microcrystals and are characterized for the strain, shape, and defect in the crystals using diffraction and microscopic studies. Through this thesis, the researchers aim to establish a method of crystal formation for paracetamol through the process of supersaturation and using isopropanol as a solvent with growth trajectory by second-order growth kinetics. This will help in the development of paracetamol crystals that are highly pure and can be used for replication in the medical industry for drug production and development.

Cathodoluminescence (CL) is a very powerful technique for studying the optical properties of semiconductor structures, especially low-dimensional structures (structures with sub-micron features). We will give a number of examples of studies, where CL has been used to study a variety of low-dimensional semiconductor structures. We will also compare the CL technique with other techniques for studies of luminescence with high spatial resolution.

Observing crystal growth processes in computer simulations

Challenges in the implementation of Nanoelectronics

In this chapter we describe some homogenization processes that may model the macroscopic properties of media whose microscopic behaviour takes lower dimensional or multi-dimensional structures into account. We will show that the method for homogenization described in Chapter 14 can be adapted to the asymptotic analysis of these functionals. In addition to this study, we also have to face the problem of the choice of a proper description of the small-scale functionals themselves, whose natural domain is not the usual Sobolev-space setting. In Sections 24.1 and 24.2 we deal with energies defined on lower-dimensional structures whose homogenization gives rise to integral functionals defined on full dimensional domains. We are going to describe two different and, to some extent, complementary approaches to this problem. We remark that while the limit analysis will be performed in detail, the description of the energies at length scale e is only outlined, leaving some details to the reader. In Section 24.3 we consider energies which describe structures as rods, plates or thin films, where one or more dimensions are negligible with respect to the others. These lower-dimensional energies are obtained by a ┌-limit procedure from (normalized) energies defined on full-dimensional domains contracting to lower-dimensional ones. We treat the case when the full dimensional structures possess in addition a (periodic) cellular structure. For the sake of illustration, we only deal with the simplified case when the period and the ‘thickness ‘ of the structure are on a comparable scale.

The glass formation ability of an alloy depends on two competing processes: glass-transition, on one hand, and crystal nucleation and growth, on the other hand. While these phenomena have been widely studied before in nearly equiatomic Cu-Zr alloys, studies are lacking for solute/solvent-rich ones. In the present work, molecular dynamics simulations show that the addition of a small amount of Zr (1-10 at. %) to Cu drastically increases the incubation time and slows down crystal growth, thus, leading to an improved glass forming ability. The crystal nucleation and growth processes of a competing face-centered cubic (FCC) Cu crystalline phase are analyzed in detail. In particular, the values of the critical cooling rate, incubation period for crystallization, and growth rate of FCC Cu crystals in these Cu-rich alloys are obtained. The growth of a supersaturated FCC Cu solid solution is found to be polymorphic at the interface (except for alloys with 9 and 10 at. % Zr) though a Zr concentration gradient is observed within growing crystals at high enough Zr content. The crystal growth rate before crystal impingement is nearly constant in all alloys, though it decreases exponentially with the Zr content. Crystallization kinetics are also analyzed within the existing theories and compared with the experimental values available in the literature.

In this chapter, we offer original solutions for crystallization devices by presenting a set of cooling devices that are upgraded models of the existing ones used in a well-known apparatus for crystal growth. Many basic ideas from these articles were used as a starting point for the creation of the new, modern multifunctional devices that may be used both as standard school laboratory tool and as industrial equipment. A number of crystal growth devices previously employed were designed to match contemporary technology level and needs for specific monocrystal growth. This led to the additional engagement on the realization of new working conditions, thereby increasing production costs. In light of this problem, while developing new forms od crystal growth apparatus we also have aimed at making the whole process as economical [1], approachable and efficient as possible. A brief review of twentieth century devices for the crystal growth from the melt [2, 3, 4] reveals widely accepted remarks on some not so good characteristics of specific apparatus components. Let’s mention Tamman’s test tube and its tip modification which is essential to the crystal germ formation, realization of suitable apparatus geometry, construction of cooler parts in order to have controlled under-cooling, some specific demands for the adequate temperature gradient. Within this chapter, we have defined certain activities conducted (with the set goal in mind) in order to improve existing and to develop new crystal growth devices. We started with a set of simple steps that allowed for the modeling and construction of school type apparatus [5, 6, 7]. Later on, we came up with original solutions and more complex devices with a number of advantages compared to the known crystal growth devices. Construction of new devices has as its ultimate goal apparatus standardization. Therefore, in a number of papers we have performed calculations that justify the use of newly designed apparatus. As a matter of a fact, in previous research, a standard and widely used approach in technology of crystal growth was to make a specific prototype of apparatus, and then, through a variety of experimentally gained data, to upgrade and improve the characteristics of crystal growth process, depending on the specific demand set for the purpose [2, 3]. That kind of approach was uneconomical regarding time consumption, and a large number of unsuccessful attempts was something one had to count on. For each specific demand a construction of an apparatus almost identical (with a slight modification only) to the one that failed was necessary. In turn, this led to significant material investments for the

The nonvolatile and continuous tuning of device conductance is the crucial requirement when developing novel materials and devices as artificial synapses. In this work we demonstrate that by exploiting various kinds of mechanisms such as ferroelectric tuning, floating gate controlling, ferroelectric-like transition in van der Waals layered 2-dimensional materials, electrokinetic phenomenon in nanofluid, etc. low dimensional devices have been successfully fabricated as memristive synapses. The advantages and challenges of using these low dimensional materials and devices as electronic synapses are illustrated with the hope of providing guidelines for the future development of this promising topic.

SiC has become the key player among wide bandgap semiconductors for power electronic applications. Since the first description of the physical vapour transport (PVT) growth process of SiC by Tairov and Tsvetkov (J. Crystal Growth, 43, 209(1978)), there has been steady progress in SiC-based crystal growth, epitaxy and device processing. The success of SiC compared to Si is related to its superior material properties such as extremely high electrical breakdown field and high thermal conductivity compared to the standard silicon counterpart. In addition, SiC device processing utilises much of the standard Si processing equipment. A major reason for the success of SiC in power electronic applications compared to other wide bandgap counterparts such as GaN, Ga2O3 and diamond is related to the availability of large diameter SiC wafer materials (150mm = standard, 200mm = developped). Bulk SiC growth is now a very well developed process with comparatively high yields. The extraordinary physical properties also include obstacles related to the strong chemical bonding and complex phase diagram of the material, which pose challenges to the growth process. Therefore, there are still a number of open questions related to the nucleation, progression and termination of the bulk growth process that require fundamental research in materials science and technology.The aim of this presentation is (i) to give an overview of the state-of-the-art PVT growth process and (ii) to discuss a current research topic dealing with the early stage of the growth process and the defect formation that can occur during the initial nucleation of SiC. We have applied 3D in-situ visualisation of the growth process using X-ray computed tomography to visualise island formation on the large seeding area. These data are related to growth process instabilities such as temperature variations during the seeding process and axial doping level changes from the seed to the newly grown crystal. Both process instabilities induce mechanical stress on the SiC lattice and act as sources for dislocation generation and multiplication. We will show a series of growth processes with varying growth parameters that shed light on the initial growth stage of SiC.As the crystal diameter of SiC increases from 150 mm to 200 mm, the results of this study become increasingly important.

4H-SiC: a material for high temperature Hall sensor

To develop deuterium-tritium (DT) cryogenic targets that meet the inertial confinement fusion (ICF) experiment requirements, the DT crystal seeding growth process needs to be controlled to obtain single crystalline DT-ice, thus reducing the crystal defects formed during crystal growth and improving ice-layering. In this paper, the close-packed hexagonal (hcp) single crystal growth mode has been established through kinetic theory of crystal growth morphology. Experimentally, the target chamber temperature is controlled to within ± 3 mK and the deuterium (D2) crystal growth process can be observed by backlit shadowgraphy. Results show that slow cooling can reduce the crystal defects significantly at the 20–100 Pa conducting helium pressure. When the cooling rate reaches 2 mK/min, two single crystal growth modes are observed with good reproducibility. Experimental results conform with the proposed hcp single crystal growth model. Compared with the results from Lawrence Livermore National Laboratory (LLNL), the methods of D2/DT single crystal growth in the cryogenic target are proposed.

The formation of wax crystals in mineral oils at low temperatures due to poor solubility of n-paraffinic components profoundly affects the oil rheology. For example, under quiescent cooling, wax crystals nucleate and grow to form macroscopic structures that cause development of a yield stress and produce rheology that is stress history dependent. The crystal formation and growth process is temperature history dependent. We have explored the effects of temperature history on the flow properties of lubricating mineral oils that do not contain performance chemical additives. The evolution of viscosity with decreasing temperature (T) is characterized by a strong increase in viscosity that occurs over a narrow 3–5 K range where the onset temperature (Tc) corresponds to that at which wax crystals become microscopically visible. Increasing the cooling rate depresses Tc and causes the activation energy in the transition region to increase. These changes correlate to a decrease in the average crystal size and a strong increase in the apparent steady state viscosity of the wax crystal dispersions at T≪Tc. Despite strong effects of stress history, we show that T≪Tc steady state flow properties are determined by the cooling rate and its apparent effects on the process occurring at the onset of crystal nucleation and growth. These results are discussed in the context of a homogeneous nucleation model for wax crystallization.The formation of wax crystals in mineral oils at low temperatures due to poor solubility of n-paraffinic components profoundly affects the oil rheology. For example, under quiescent cooling, wax crystals nucleate and grow to form macroscopic structures that cause development of a yield stress and produce rheology that is stress history dependent. The crystal formation and growth process is temperature history dependent. We have explored the effects of temperature history on the flow properties of lubricating mineral oils that do not contain performance chemical additives. The evolution of viscosity with decreasing temperature (T) is characterized by a strong increase in viscosity that occurs over a narrow 3–5 K range where the onset temperature (Tc) corresponds to that at which wax crystals become microscopically visible. Increasing the cooling rate depresses Tc and causes the activation energy in the transition region to increase. These changes correlate to a decrease in the average crystal size and a st...

Crystal growth melt flow control by means of magnetic fields

One-step solution synthesis of urchin-like ZnO superstructures from ZnO rods

Blends of ethylene–methyl acrylate–glycidyl methacrylate terpolymer (E‐MA‐GMA, a random terpolymer) and polycarbonate (PC) were prepared in a Haake torque rheometer and the rheological properties, phase morphology, and thermal behavior were investigated. The graft reactions of PC terminal hydroxyl groups with the epoxy groups of E‐MA‐GMA and the in situ formation of the E‐MA‐GMA‐g‐PC copolymers at the interface were illustrated by the improved mixing torque and melt viscosity in E‐MA‐GMA/PC blends. Typical variation and significant deformation of the dispersed phase was observed in E‐MA‐GMA/PC blends with different composition, where PC was the matrix. With the E‐MA‐GMA content increasing, a complex co‐continuous phase structure with some dispersed E‐MA‐GMA particles wrapped in the continuous PC phase was present, indicating strengthened interfacial adhesion. When the E‐MA‐GMA content was higher than the PC component, fibrous structure of the dispersed PC phase in the E‐MA‐GMA matrix was caused by shear flow and interfacial interaction. DSC studies showed that the melting point of E‐MA‐GMA shifted to lower temperature with the increase of PC content, indicating that the enhanced interaction and graft structure hindered the process of crystallization and crystal growth.