@phdthesis{Lange2021, author = {Lange, Felix}, title = {MBE growth and investigation of Si, Ge, and SiₓGe₁₋ₓ nanowires}, doi = {10.26127/BTUOpen-5751}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:co1-opus4-57510}, school = {BTU Cottbus - Senftenberg}, year = {2021}, abstract = {Within this work, the growth of out-of-plane Si and SiᵪGe₁₋ᵪ and in-plane Ge nanowire have been investigated. For this purpose, nanowires have been grown ccording to the vapor-liquid-solid mechanism by means of molecular beam epitaxy with Au as metallic solvent on Si(111) and nano-structured Si(001)/SiO₂ substrates. Each stage from surface preparation to final nanowire growth has been examined in order to determine relevant parameters which influence the nucleation process nanowire elongation. Particular attention has been given to describe the variation of Au droplet size distribution on Si(111) to evaluate the subsequent nanowire growth and to obtain optimal growth conditions for an site-selective nucleation on the nano-structured substrates. Due to increased surface diffusion velocity with raising temperature, the droplet diameter distribution shifts from a positive skewed distribution with a high fraction of smaller droplets to a negative skewed distribution with a high fraction of larger droplets. The temperature dependency of the most probable droplet diameter and the number of Au droplets per area has been determined, which can be applied to predict the resulting nanowire diameters. The out-of-plane Si and SiᵪGe₁₋ᵪ on Si(111) predominantly grow along the surface normal, exhibiting the characteristic sawtooth-like sidewall faceting. During the nanowire formation, the Au surface diffusion velocity increased and causes an increase of the most frequently observed diameter. Furthermore, the total number of droplets/nanowires decreases by a constant factor due to an increase of the contact angle during Si/Ge deposition. The specific diameter range for an possible nanowire formation is reduced by the incorporation of Ge into SiᵪGe₁₋ᵪ nanowires. To obtain a regular and uniform nanowire growth, a nano-structured substrate consisting of Si(001) terminated pillars surrounded by a SiO₂ matrix has been utilized. The initial growth of Ge nanowire starting from Si-Au droplets with SiᵪGe₁₋ᵪ nucleation from ternary alloy is discussed from a thermodynamic point of view and a model based on the Si-Ge-Au ternary phase diagram has been developed to predict the SiᵪGe₁₋ᵪ concentration gradient in the nanowire base. The fully relaxed in-plane Ge nanowires occur within one of the four distinct in-plane ⟨110⟩ directions and nanowires are mainly bounded by two 55° inclined {111} facets and a less pronounced planar (001) top facet. High-resolution scanning X-ray diffraction microscopy reveals a slightly tilted growth of individual nanowires with respect to each other, causing an abrupt change in the orientation at junction points of interconnected nanowires.}, subject = {MBE; Silicon; Germanium; Nanowires; Epitaxy; MBE; Silizium; Germanium; Nanodr{\"a}hte; Epitaxie; Nanodraht; Molekularstrahlepitaxie; Germanium; Silicium; Tropfengr{\"o}ßebestimmung}, language = {en} } @phdthesis{Scherbahn2018, author = {Scherbahn, Vitali}, title = {Towards ultrasensitive SPR-based sensing: self-referencing and detection of single nanoparticles}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:co1-opus4-47119}, school = {BTU Cottbus - Senftenberg}, year = {2018}, abstract = {Surface plasmon resonance (SPR) and its extensions, surface plasmon resonance imaging (SPRi) and surface plasmon resonance microscopy (SPRM) both enabling visualization of the sensor surface, belong to classical, indispensable highly sensitive and robust optical (bio)analytical techniques to study affinity processes on a surface. Nevertheless, SPR and SPRi/SPRM undergo a continuous development in regard to the improvement of sensitivity. The main challenge in this direction is attributed to the separation of signals due to the binding of analytes and those due to the bulk effect. The main task of the present thesis was to apply different strategies to improve the performance of SPR sensing. Within this scope, two main objectives were pursued: (1) implementation and realization of a so-called internal referencing towards suppression of the bulk effect leading to an improvement and optimization of the signal-to-noise ratio (SNR) and (2) application of wide-field (WF)-SPRM to detect, to visualize and to characterize single nanoparticles adsorbed to modified surfaces. The first objective of this thesis comprises the realization of three different internal-referencing approaches. In the first approach, a self-referencing effect based on arbitrarily distributed micro-patterned self-assembled monolayer (SAM) containing sensing and referencing spots was realized. Measurements of classical antigen-antibody-interaction resulted in a 10-fold improvement of the SNR by suppression of the bulk effect and the corresponding microfluctuations of the bulk temperature. The application of the second internal-referencing-approach, ionic referencing, acting as an assessment of patterned SAM was realized using electrolytes with a high molar refraction of either anions or cations to micro-patterned SAM combined with WF-SPRM as detecting technology. As a result, successful, unobtrusive visualization and spatial distinction of micro-patterned surfaces was shown. Unlike visualization of micro-scaled surface areas, the application of spatio-temporal referencing in WF-SPRM, as a third type of internal referencing, enables to detect, moreover to visualize and localize, smallest changes in refractive index near/on the sensor surface. In that sense, the second objective of this thesis was dedicated to the application of the WF-SPRM technology combined with spatio-temporal referencing to detect, to visualize, to quantify and to characterize single nanoparticles adsorbed to the sensor surface; here, nanoparticles act as analyte species. Based on a sophisticated image analysis, successful detection and characterization of single nanoparticles in complex media such as wine, juice and sun cream was performed. Besides being a powerful solution for nanoparticles analytics, the WF-SPRM technology represents a base to develop novel, ultra-sensitive and fast (bio)sensing platforms. Within this scope, enzyme-assisted generation of nanoparticles was studied.}, subject = {Surface plasmon resonance microscopy; Self-referencing; Single nanoparticles; Ezymatic synthesis of nanoparticles; Surface patterning; Enzymvermittelte Synthese von Nanopartikeln; Selbstreferenzierung; Einzelne Nanopartikel; Oberfl{\"a}chenstrukturierung; Oberfl{\"a}chenplasmonenresonanzmikroskopie; Oberfl{\"a}chenplasmonenresonanz; Oberfl{\"a}chenstruktur; Biosensor; Nanopartikel}, language = {en} } @phdthesis{Reis2021, author = {Reis, Bruno Henrique}, title = {Development of a novel thermodynamic database for salt systems with potential as phase change materials}, doi = {10.26127/BTUOpen-5590}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:co1-opus4-55902}, school = {BTU Cottbus - Senftenberg}, year = {2021}, abstract = {Research on energy saving technologies surged in the last decades. One especially relevant technology regards thermal energy storage via phase change materials, or PCM. These materials function as regenerative thermal batteries that can absorb and release thermal energy via the latent heat associated with a phase change, while temperature is kept constant. The advantage of this technology is that due to the latent heat effect the energy density is very high, which reduces the required size of the medium and makes it easier to be coupled with heat loss sources, both in industrial and household applications. The challenge lies, however, in identifying correct PCMs for specific operation temperatures. The goal of the thesis is, then, to develop a novel thermodynamic database that describes the thermodynamic properties of salt mixtures with potential as phase change materials, both for high (up to 800 ℃) and low temperature (up to 100 ℃) applications; and, then, to perform a screening to identify potential PCM compositions in the database. The database is created with FactSage, a Calphad software, and the systems covered are the CaCl₂-Ca(NO₃)₂-KCl-KNO₃-NaCl-NaNO₃, for high temperature PCMs, and the hydrated Mn(NO₃)₂-H₂O, Zn(NO₃)₂-H₂O, MgSO₄-H₂O and ZnSO₄-H₂O for low temperature PCMs. The liquid solution in all systems is modelled with the non-ideal associates model and, therefore, no aqueous solution model is required. The experimental data used for the assessments come from the literature and from new measurements performed by the partners of the PCM-Screening project (FKZ 03ET1441). A new program called DataOptimizer has been developed to assist with the optimisation of thermodynamic databases. Relying on the ChemApp software and the NOMAD optimizer, DataOptimizer overcomes many shortcomings of similar database optimisation programs. A graphical user interface featuring a real-time plotting output is also implemented, which allows for a much easier and user-friendly experience. Details about the implementation and features of the program are given. Finally, the identification of PCM candidates is performed using both phase diagrams calculated with FactSage and a new numerical screening algorithm, which relies on ChemApp. The screening algorithm proves to be capable of identifying eutectics in multicomponent systems automatically without the need for phase diagrams. As a result, twenty-two PCM candidates are identified for high temperature applications within the anhydrous system and, fourteen candidates, for low temperature applications within the hydrated systems.}, subject = {Phase change materials; Salt mixtures; Calphad; Thermodynamic database; Screening of eutectics; Phasenwechselmaterialien; Salzmischungen; Calphad; Thermodynamische Datenbank; Screening von Eutektika; Latentw{\"a}rmespeicher; Phasen{\"u}bergangswerkstoff; Salz; Gemisch; Thermochemie; Phasendiagramm; Eutektikum}, language = {en} } @phdthesis{Groenke2020, author = {Gr{\"o}nke, Martin}, title = {Synthesis and characterization of layered transition metal trihalides MCl₃ (M = Ru, Mo, Ti, Cr) and CrX₃ (X = Cl, Br, I)}, doi = {10.26127/BTUOpen-5282}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:co1-opus4-52828}, school = {BTU Cottbus - Senftenberg}, year = {2020}, abstract = {The investigation of novel structure-to-property relations of many transition metal trihalides MX₃ by downscaling to promising monolayer is still pending. However, the production of two-dimensional MX₃ sheets that are both high crystalline and thin is an experimental challenge. This thesis is focused on the rational synthesis planning and the derived targeted preparation of thin MX₃ nanosheets (≤ 100 nm) on suitable substrates by chemical vapor transport (CVT) as well as their characterization by complementary analytical methods. CVT of nanosheets directly on substrates benefits of low timescales, less material consumption and only few structural distortions. For the determination of optimal growth conditions, the CVT processes of investigated compounds were initially simulated by using the Calphad method (program package TRAGMIN). Thus, the occurring transport efficient gas species and temperature dependent, dominating vapor transport equilibria were calculated to optimize the growth process in a direct and straightforward way. Based on prior simulation results single crystalline sheets of MCl₃ (M = Ru, Mo, Ti, Cr) and CrX₃ (X = I, Br, Cl) were successfully prepared at temperatures between 573 - 1023 K on YSZ (yttrium stabilized zirconia) or sapphire substrates. The adjustable CVT parameters (transport duration, temperatures or weighed starting material) were optimized with respect to the targeted synthesis of either bulk or nanosheets at substrates. Microsheets with thicknesses of less than 4 μm (α-TiCl₃) and about 20 nm thin nanosheets (α-RuCl₃, CrCl₃ and CrI₃) down to ultrathin flakes (≈ 3 nm, α-MoCl₃ and CrBr₃) were obtained by CVT. As a highlight, monolayers of α-RuCl₃ and CrCl₃ were isolated successfully by means of a subsequent delamination. The MX₃ sheets morphology and dimension was described by optical and electron microscopy, highlighting their two-dimensional nature. By several X-ray spectroscopy and diffraction techniques the desired composition (M:X = 1:3), high crystallinity and phase-purity of thick and thin MX₃ platelets was confirmed subsequently. With respect to MX₃ nanosheets a slight increase (α-RuCl₃, α-MoCl₃ and CrBr₃) or decrease (CrCl₃) in phonon energies was observed in comparison to their bulk counterparts. The magnetic properties of CrCl₃ micro- and nanosheets were determined to be solely ferromagnetic and thus different than those of the bulk samples. Finally, the structure-to-property relations were investigated at a first example. The catalytic properties of α-TiCl₃ microsheets were investigated by gas-phase polymerization of ethylene. By downscaling the catalysts thickness by CVT, we obtained an activity improvement of 24 \% in comparison to bulk α-TiCl₃.}, subject = {Crystal; Thermodynamic; Vapor phase; Synthesis; Simulation; Kristall; Thermodynamik; Gasphase; Synthese; Simulation; Syntheseplanung; {\"U}bergangsmetallverbindungen; Metallhalogenide; Rutheniumhalogenide; Molybd{\"a}nhalogenide; Gasphasenreaktion}, language = {en} } @phdthesis{Putze2026, author = {Putze, Philip}, title = {Screw dislocation-driven growth of 2D transition metal dichalcogenide nanostructures by chemical vapour transport}, doi = {10.26127/BTUOpen-7302}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:co1-opus4-73028}, school = {BTU Cottbus - Senftenberg}, year = {2026}, abstract = {Screw dislocation-driven growth of nanostructures of two-dimensional (2D) transition metal dichalcogenides (TMDs) can feature special physical behaviour like unconventional superconductivity and Moir{\´e} excitons. Three of the outstanding representatives of TMDs are WSe₂, MoSe₂, and WS₂ as they can exhibit intriguing new size- and shape-depended properties compared to its bulk counterpart. Crystal growth control in nanostructures is central for exploiting their structure-related properties. However, bottom-up syntheses of 2D TMDs usually contain 'trial and error' approaches. This work addresses the rational synthesis planning and realizing for the binary systems W:Se, Mo:Se and W:S to achieve nanosized flakes by chemical vapor transport (CVT). For that purpose, key parameters for the CVT were modelled based on thermodynamic datasets and transferred into actual experiments. Regarding WSe₂, crystal growth by CVT under addition of SeCl₄ succeeds for bulk-WSe₂ from 900 °C to 820 °C with a dwell time of 72 h. High-crystalline, right-handed spirals with step heights of around 10 nm are obtained from 850 °C to 800 °C with a dwell time of 60 min, while left-handed spirals occur from 915 °C to 860 °C. These characteristics are linked to theoretical considerations. The terrace widths correlate linearly to the supersaturation condition during the crystal growth, which is represented by the temperature-depended equilibrium constant. Chirality of WSe₂ with screw dislocation-driven growth was investigated by circular-polarised Raman Spectroscopy and showed an intensity increase up to 29 \% for the E¹₂ᶢ mode. For MoSe₂, the primary choice of substrate dictates the resulting crystal shapes. Under essentially identical growth conditions, i.e., 710 °C → 685 °C and a dwell time of 30 min, SiO₂@Al₂O₃(0001) substrates can be used for the controlled precipitation of both hexagonally and triangular shaped nano-MoSe₂. Compared to that, SiO₂@Si(100) substrates enable the growth of predominantly triangular crystals. Individual steps vary from 0.9 nm to 2.9 nm. Nano-MoSe₂ exhibits a twist angles sequence of ca. 5°. Additionally, a twist angle of 19° can be obtained with respect to the [010] zone axis. Supertwisted WS₂ flakes occur from 810 °C to 770 °C under addition of WCl₄(s) and a dwell time of 40 min, while the bulk counterpart in the 2H modification can be obtained from 1030 °C to 950 °C with a dwell time of 96 h. Tailored non-Euclidean Si(100) substrate surfaces with seeded SiO₂ nanoparticles induce WS₂ spirals with twist angles of - 19° for right-handed spirals with individual step heights in the monolayer range.}, subject = {Screw dislocation; 2D materials; CVT; Rational synthesis; Chiral nanostructures; Schraubenversetzung; 2D Materialien; Rationale Synthese; Chirale Nanostrukturen; Schraubenversetzung; Zweidimensionales Material; Chiralit{\"a}t ; {\"U}bergangsmetalldichalkogenide; Syntheseplanung; Wolfram; Molybd{\"a}n; Selen; Schwefel}, language = {en} }