1,721,679 research outputs found
Thermoelectric Performance of IV-VI Compounds with Octahedral-Like Coordination: A Chemical-Bonding Perspective
No full textThermoelectric materials provide a challenge for materials design, since they require optimization of apparently conflicting properties. The resulting complexity has favored trial‐and‐error approaches over the development of simple and predictive design rules. In this work, the thermoelectric performance of IV–VI chalcogenides on the tie line between GeSe and GeTe is investigated. From a combination of optical reflectivity and electrical transport measurements, it is experimentally proved that the outstanding performance of IV–VI compounds with octahedral‐like coordination is due to the anisotropy of the effective mass tensor of the relevant charge carriers. Such an anisotropy enables the simultaneous realization of high Seebeck coefficients, due to a large density‐of‐states effective mass, and high electrical conductivity, caused by a small conductivity effective mass. This behavior is associated to a unique bonding mechanism by means of a tight‐binding model, which relates band structure and bond energies; tuning the latter enables tailoring of the effective mass tensor. The model thus provides atomistic design rules for thermoelectric chalcogenides
Growth of textured chalcogenide thin films and their functionalization through confinement
Full text linkA number of chalcogenides show a remarkable portfolio of properties, which enables a plethora of applications including thermoelectric power generation and phase change material (PCM)-based information storage. PCMs exhibit fast and reversible switching between two (typically, amorphous and crystalline) states, characterized by different optical and electrical properties. In the last decade, textured chalcogenide thin films have been investigated to develop a better understanding of structure - property relationships, improve material performance and understand how these properties change upon reducing film thickness. In this review, the present knowledge concerning the textured growth is summarized, focusing on films of GeTe, Sb2Te3, and GeSbTe compounds. In particular, the impact of different deposition methods, substrate surface modifications, and methods to influence film formation is reviewed. In the second part of this review, confinement effects are discussed. Surprisingly pronounced changes in atomic arrangement, that affect film properties, and device performance are presented. These changes are attributed to the unconventional bonding mechanism in these chalcogenides coined metavalent bonding, which is trongly affected by the confinement. Bonding becomes covalent-like in the two-dimensional limit, whereas Metavalent Bonding emerges for thicker films, where electron delocalization is increased This explains the pronounced property changes with film thickness.In this review, the growth of textured chalcogenides, such as GeTe and Sb2Te3 with physical deposition methods is covered and different growth schemes presented in literature are evaluated. Furthermore, effects of reducing film thicknesses to ultrathin films are discussed. Such confinement leads to increased structural distortions, which enables tailoring of film properties by precise film thickness controlimage (c) 2024 WILEY-VCH Gmb
Chalcogenide Thermoelectrics Empowered by an Unconventional Bonding Mechanism
Full text linkThermoelectric materials have attracted significant research interest in recent decades due to their promising application potential in interconverting heat and electricity. Unfortunately, the strong coupling between the material parameters that determine thermoelectric efficiency, i.e., the Seebeck coefficient, electrical conductivity, and thermal conductivity, complicates the optimization of thermoelectric energy converters. Main-group chalcogenides provide a rich playground to alleviate the interdependence of these parameters. Interestingly, only a subgroup of octahedrally coordinated chalcogenides possesses good thermoelectric properties. This subgroup is also characterized by other outstanding characteristics suggestive of an exceptional bonding mechanism, which has been coined metavalent bonding. This conclusion is further supported by a map that separates different bonding mechanisms. In this map, all octahedrally coordinated chalcogenides with good performance as thermoelectrics are located in a well-defined region, implying that the map can be utilized to identify novel thermoelectrics. To unravel the correlation between chemical bonding mechanism and good thermoelectric properties, the consequences of this unusual bonding mechanism on the band structure are analyzed. It is shown that features such as band degeneracy and band anisotropy are typical for this bonding mechanism, as is the low lattice thermal conductivity. This fundamental understanding, in turn, guides the rational materials design for improved thermoelectric performance by tailoring the chemical bonding mechanism
Phase change materials for non-volatile electronic memories
No full textStarting from a brief introduction into the physics of phase change materials their applications in the field of data storage are reviewed. Without the latter there would certainly be less attention paid to the topic today, even in basic research. For years now, optical data storage media based on phase change materials have been successfully produced for the mass market. Despite its maturity there are still new ideas on how to further improve this technology in order to stay competible with other storage media in the future. In contrast, for the application of phase change materials in electronic memories the situation is completely different. Although proposed decades ago, today it is still a rather young and promising technology. In this work both applications are analyzed with respect to their requirements to the incorporated phase change material, with an emphasis on the second one. For optical storage and especially in the case of electronic memories the author arrives at the conclusion, that the crystallization kinetics of phase change materials is the most urgent and fundamental problem that needs to be solved. The emphasis on a deep and quantitative understanding of this phenomenon implicitely criticizes the popular approach of studying the switching of electronic cells for phase change based memories without an in-depth research of the crystallization kinetics. After this identification of the research objective, the result of a thorough review of the literature on the theory of crystallization is presented. The understanding of glass formation turns out to be extremely important, since it deals with the stability of an amorphous solid or undercooled liquid against structural reconfiguration. Consequently, a separate chapter is dedicated to the theory of glass formation. Based on the knowledge of the theoretical connections between glass transition and crystallization it is investigated how meaningful a calculation of the enthalpy of atomization is for a prediction of stoichiometric trends not only of glass transition, but also of crystallization. The sparce experimental evidence on the glass transition temperature of phase change materials is compared to the calculated enthalpies. The same is done for a series of measurements of crystallization. These comparisons show that the proposed strategy works well for predicting the influence of a stoichiometric variation of a phase change material on its stability against crystallization. The author's critical review of the literature on crystallization kinetics reveals that the widely used classical crystallization theory still lacks a rigorous experimental prove of its validity. The latter is difficult, because the multitude of quasi-free parameters generally ensures a good mathematical agreement between theory and the often very limited experimental data. Such a check of validity of the theory is especially challenging for phase change materials: In a wide range of high temperatures crystallization proceeds so fast, that until now an experimental quantification of nucleation rate and crystal growth velocity was only possible in a rather limited regime of low temperatures. The extrapolation of such data over the whole temperature range up to the melting temperature by application of the equations provided by the classical theory is assessed to be too uncertain to be trusted. To close that gap with experimental evidence and to advance therewith towards a disentanglement of electronic and thermal effects involved in the switching of an electrical cell, a new experimental setup has been designed and realized. It combines laser induced annealing experiments with the capability to apply and measure fast electrical pulses. The implementation of a control of the sample's base temperature is an additional, valuable component. Each of the sections of the new setup on its own is already a sophisticated tool, that enables its user to investigate phase change materials on very short time scales. Examples for this are unprecedented laser experiments that have been performed by the author. Some of those innovatively separate crystal nucleation and growth. Others demonstrate a path towards a quantitative measurement of crystallization kinetics in the melt-quenched amorphous state. The latter is highly relevant for technology, since it is that amorphous phase, that is realized in applications. But the new setup is more than just the sum of its parts. The combination of optical, electrical and thermal experiments opens up a wide field of new possibilities. This is indicated by the demonstration of an electrical experiment for the investigation of the threshold-switching effect, the phenomenon describing an abrupt breakdown of the resistivity of the amorphous phase upon application of a critical electric field. The initialization of the sample by laser annealing allows for a clean'' experiment in which the starting conditions are not dependent on the property of interest itself, in this case the electrical behaviour. Such a successive approach from purely thermal experiments towards the testing of the realistic but complex cells of a phase change based electronic memory is necessary to stepwise synchronize the numerical simulations. This strategy is essential for achieving a deep understanding of the physical processes involved in the switching of such cells. Beyond these technologically important measurements the new setup is pioneering for a multitude of further optical and electrical experiments that are likely to make valuable contributions to the research of the physical properties of phase change materials. Examples for such experiments are proposed at the end of this work
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Electronic and optical properties of phase change alloys studied with ab initio methods
No full textIn this work the correlation between structural, electronic and optical properties of so-called phase-change materials is studied. Employing methods of computational physics, in particular density functional theory and many-body perturbation theory this work reveals the correlation between the change of the local atomic structure upon amorphization and the change of the electronic properties. In the phase change alloy Ge1Sb2Te4 the germanium atoms, which are octahedrally coordinated in the crystalline phase, switch to tetrahedrally coordinated positions in the amorphous phase. Calculations with density functional theory show, that such a local arrangement is energetically favorable while it correctly reproduces the experimentally observed density change upon amorphization. This structural change leads to pronounced changes in the electronic levels. In particular electrons of the tellurium atoms sharply decrease in energy. While they significantly contribute to the density of states at the Fermi energy in the crystalline phase, they are well below the Fermi level in the amorphous state. This decrease in energy results in an opening of the band gap in the amorphous state. This is in contrast to conventional semiconductors, which exhibit a reduction of the band gap energy in the amorphous phase. This widening of the band gap in the amorphous phase is of great importance for the future application of phase change materials in electronic data storage. Besides the electronic properties the optical properties are of great importance for phase change materials. The large difference of the optical absorption in the amorphous and the crystalline phase, which is not observed in conventional covalent semiconductors such as Si or GaAs, is not yet understood. In order to study this optical contrast between the two structural states, calculations of the optical absorption of GeTe and Ge1Sb2Te4 within time-dependent density functional theory and many-body perturbation theory have been performed within this work. The results reveal the effect of the change in structural and electronic properties upon amorphization on the optical properties. The optical absorption becomes broader and flatter in the amorphous state, which is confirmed by spectroscopic measurements using Fourier-transform infrared spectroscopy and ellipsometry. Thus the calculations reveal for the first time the correlation between the local structure and the optical absorption. Finally a detailed analysis of the calculated data leads to an explanation for the unusual change in the absorption upon amorphization. Usually it is assumed, that the amorphization of covalent semiconductors leads to the creation of defect states in the gap and thus to a smearing of the electronic states which in turn results in a moderate change of the optical properties. The calculations presented in this work show that in GeTe as well as in Ge1Sb2Te4 the optical contrast cannot solely be explained by this effect. In both alloys the matrix elements of the optical transitions significantly change upon amorphization, providing an important contribution to the large optical contrast. The fact, that the strength of the matrix elements and of their change upon amorphization is different for the two alloys, indicates that this important property of phase change materials can be adjusted and controlled by a systematic selection of the stoichiometry. Thus this result represents an important contribution to a systematic material optimization of phase change alloys
Growth, structure and morphology of organic thin films
No full textOver the past two decades, organic semiconductors have emerged as a technologically important class of electronic materials. Promising applications include organic field effect transistors (OFETs), organic light-emitting devices (OLEDs), organic lasers, and photovoltaic cells. These devices have in common that they are based on organic thin films, and that they are very sensitive to the order of these films. Contrary to traditional inorganic electronic materials, organics are characterized by complex and covalently bonded building blocks (molecules) that are held together by weak van der Waals (vdW) interactions. The morphology and growth of organic films on insulating substrates are of particular interest as this configuration is used in Organic Thin Film Transistors (OTFTs). Planar Aromatic Hydrocarbons (PAHs) typically have a broad intermolecular interaction potential energy dominated by vdW interactions. Thus, the molecule-substrate interaction could play a significant role in determination of the subsequent crystalline structure. They have a simple and planar structure and they are considered to be used as a prototype of polycyclic aromatic hydrocarbons due to their relatively regular molecular shape. However, for perylene there is still a lack of systematic studies on the morphology and the structure of perylene thin film. In this thesis, perylene have been used as an organic semiconductor material. The necessity to understand the growth mechanism and fabrication of a highly crystalline film led to the deposition of perylene on different substrates. These samples were deposited with different deposition rates and different thicknesses. The influence of these deposition parameters and substrate has been investigated by Atomic Force Microscopy (AFM) and X-Ray Diffraction (XRD) techniques. AFM has been employed to investigate the surface morphology of samples in real space and XRD has been used to determine the crystalline structure of the thin-film system. A metal oxide bottom layer and low deposition rate (2 °A/s) was found to lead to a well-ordered perylene layer. Furthermore, for OLED applications it is necessary to have a smooth and amorphous film because in such a device, the highest possible quantum yield is desirable. This yield depends upon the probability of radiative electron-hole recombination which is the highest for amorphous materials, where the electron and hole mobilities are low. Organic Vapor Phase Deposition (OVPD) and Vacuum Thermal Evaporation (VTE) were chosen as deposition methods to produce amorphous film. Subsequently, XRD and XRR have been employed to investigate film morphology and structural properties. The thermal stability of the film was examined by In situ heating-XRR technique
Subantrale Augmentation mit xenogenen und alloplastischen Knochenersatzmaterialien in Kombination mit PRP : eine klinische Studie mit Split-Mouth-Design
No full textThe influence of energetic bombardment on the structure formation of sputtered zinc oxide films : development of an atomistic growth model and its application to tailor thin film properties
No full textThe focus of this work is the investigation of the growth of zinc oxide (ZnO) thin films. This material has attained increasing scientific interest during the last decade. Particularly, doped zinc oxide films have become the material of choice in the fabrication of transparent electrodes for silicon thin film solar cells. Another market segment where zinc oxide films are utilized is the fabrication of low-emissivity architectural glazing. The functionality of these energy saving windows arises from a combination of high optical reflectivity in the infrared spectral region with optimum transparency in the visible regime. This prerequisite could be easily achieved with an arbitrarily complex multi-layer stack. Competition however requires minimization of production costs. Therefore, typically one or two very thin silver films in combination with a minimum number of anti-reflective oxide layers are utilized. Maximization of production efficiency requires tweaking each single layer for optimum performance, which is particularly true for the silver films. For this reason, zinc oxide films are utilized as seed layers for silver film growth since their close epitaxial relationship significantly promotes the formation of a well-ordered silver crystal structure with a preferred orientation. Thus, optimizing silver layer performance requires mastering zinc oxide film growth. A common feature of these applications of zinc oxide thin films is that if material properties can be significantly improved, either the device efficiency can be maximized and/or the production costs can be minimized. It is therefore highly desirable to establish a thorough understanding of zinc oxide film growth. Particularly, this understanding must include the influence of energetic ion bombardment, a feature which is inherent in the coating process. For the applications mentioned above zinc oxide is most commonly deposited in large scale onto amorphous float glass substrates by a deposition process far away from thermodynamic equilibrium: sputter deposition. Consequently, films often exhibit kinetically controlled structures. However, a self-texturing mechanism of zinc oxide that might originate from thermodynamics typically leads to high structural order with increasing film thickness. In spite of that mechanism there is unused potential since films are often weakly textured in the initial growth stage, a fact which also limits the achievable maximum in the structural order of thick films. Particularly in the fabrication of low-emissivity coatings, where very thin zinc oxide films are utilized, device performance critically depends on the structural quality obtained in the early growth stage of zinc oxide. It is therefore vital to understand how different process parameters affect structure formation. In the literature on zinc oxide thin films it is comprehensively portrayed that films often exhibit structural inhomogeneities along the growth direction if grown on amorphous (non-epitaxial) substrates. Also, the detrimental influence of highly energetic oxygen ion bombardment on film growth in general is extensively discussed. However, literature on possible positive influences of tailored ion bombardment is rare; just as literature on possible correlations between different growth stages of the zinc oxide films and the extent of structural modification/damage caused by ion bombardment. Closing this gap is the scope of this work. It will be demonstrated that with a modified, ion beam assisted sputtering (IBAS) process, zinc oxide films can be deposited which exhibit a markedly improved crystalline order. Furthermore, it will be demonstrated that intense energetic oxygen ion bombardment can be utilized to change film texture from the typical (002)-self-texture to an a-axis texture where the (002)-planes are perpendicular to the substrate surface. An understanding of the underlying mechanisms will be developed which also facilitates a more detailed understanding of the action of ion bombardment during zinc oxide film growth. It will be shown that zinc oxide films are susceptible to the influence of ion bombardment particularly in the nucleation regime of growth and that this finding is generally true for all observed structural changes induced by ion bombardment with various species, energies and flux densities. These findings will therefore allow answering fundamental questions regarding the mechanisms governing structural evolution of zinc oxide thin films, which is also the scope of this work. It will be demonstrated not only that the initial growth stage plays an important role in the formation of a preferred growth orientation but also that the action of texture forming mechanisms in subsequent growth stages is comparatively weak
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