@misc{FroeschkeSchrothSteineretal., author = {Froeschke, Samuel and Schroth, Karl-Georg and Steiner, Udo and Popov, Alexey and Schiemenz, Sandra and Wolf, Daniel and Giebeler, Lars and Gr{\"a}ßler, Nico and B{\"u}chner, Bernd and Schmidt, Peer and Hampel, Silke}, title = {Understanding the chemistry of 2D rhodium trihalide solid solutions: tuning of optical properties and nanocrystal deposition}, series = {2D Materials}, volume = {10}, journal = {2D Materials}, number = {3}, issn = {2053-1583}, doi = {10.1088/2053-1583/acd012}, pages = {1 -- 12}, abstract = {In the search for novel 2D materials with potentially valuable properties, such as a tunable band gap for optoelectronic or catalytic applications, solid solutions hold the potential to significantly expand the inventory of available 2D nanomaterials. In this study, we present for the first time the synthesis of such 2D rhodium trihalide solid solutions: RhBrxCl3-x and RhBrxI3-x. We use thermodynamic simulations and simultaneous thermal analysis to predict conditions for their rational synthesis and to investigate suitable chemical vapor transport (CVT) parameters for these solid solutions. The evolution of the lattice parameters was investigated by powder x-ray diffraction, showing an isostructural relationship of the synthesized compounds and only minor deviation from Vegard's law. The optical band gap of these materials can be tuned in an energy range from 1.5 eV (RhCl3) to 1.2 eV (RhI3) by choosing the composition of the solid solution, while the samples also exhibit photoluminescence in similar energy ranges. Ultimately, the successful deposition of bulk as well as ultrathin 2D nanocrystals of RhBrxCl3-x by CVT from 925 °C to 850 °C is shown, where the composition of the deposited crystals is precisely controlled by the choice of the starting composition and the initial amount of material. The high quality of the obtained nanocrystals is confirmed by atomic force microscopy, high resolution transmission electron microscopy and selected area electron diffraction. For RhBrxI3-x, the CVT from 900 °C to 825 °C is more difficult and has only been practically demonstrated for an exemplary case. According to the observed properties, these novel solid solutions and nanocrystals show a great potential for an application in optoelectronic devices.}, language = {en} } @misc{GroenkePohfleppSchmidtetal., author = {Gr{\"o}nke, Martin and Pohflepp, Danny and Schmidt, Peer and Valldor, Martin and Oswald, Steffen and Wolf, Daniel and Hao, Qi and Steiner, Udo and B{\"u}chner, Bernd and Hampel, Silke}, title = {Simulation and synthesis of alpha-MoCl₃ nanosheets on substrates by short time chemical vapor transport}, series = {Nano-Structures \& Nano-Objects}, journal = {Nano-Structures \& Nano-Objects}, number = {19}, issn = {2352-507X}, doi = {10.1016/j.nanoso.2019.100324}, pages = {1 -- 7}, abstract = {Alpha-molybdenum(III) chloride (MoCl₃) belongs to layered van-der-Waals materials, which are in focus to exhibit interesting properties due to their weak chemical and magnetic interactions. Especially the structure of alpha-MoCl₃ has been discussed in terms of symmetry breaking dimerization of Mo atoms at room temperature, which might led to exotic ground states. By exploiting the 2D materials characteristics, an investigation of physical properties on the nanoscale is intended. We herein demonstrate the probably first approach to synthesize phase pure, as-grown alpha-MoCl₃ few-layer nanosheets by means of a pure short time chemical vapor transport (CVT) process. Vapor growth benefits from a one-step deposition of high crystalline alpha-MoCl₃ nanosheets without stacking faults on a substrate. Thus, mostly applied subsequent delamination, associated with the introduction of structural defects, becomes redundant. According to the CVT process thermodynamic simulations of gas phase equilibria have been performed and the synthesis conditions could be optimized based on the calculation results. By CVT the as-grown nanolayers are deposited on sapphire (Al₂O₃) substrates by applying a temperature gradient of 70 K from 743 K to 673 K. Single crystalline sheets with thicknesses ≤75 nm down to five layer (3 nm) could be obtained by using a pure CVT process. According to the deposited nanostructures we approve the desired composition, morphology, phase purity and high crystallinity by using several microscopy and spectroscopy techniques. Furthermore, we show micro-RAMAN measurements which hint at a slight increase in phonon energies for nanosheets in comparison to the corresponding bulk phase.}, language = {en} } @misc{GroenkeBuschbeckSchmidtetal., author = {Gr{\"o}nke, Martin and Buschbeck, Benjamin and Schmidt, Peer and Valldor, Martin and Oswald, Steffen and Hao, Qi and Lubk, Axel and Wolf, Daniel and Steiner, Udo and B{\"u}chner, Bernd and Hampel, Silke}, title = {Chromium Trihalides CrX₃(X = Cl, Br, I): Direct Deposition of Micro- and Nanosheets on Substrates by Chemical Vapor Transport}, series = {Advanced Materials Interfaces}, volume = {6}, journal = {Advanced Materials Interfaces}, number = {24}, issn = {2196-7350}, doi = {10.1002/admi.201901410}, pages = {11}, abstract = {The experimental observation of intrinsic ferromagnetism in single layered chromium trihalides CrX₃ (X = Cl, Br, I) has gained outstanding attention recently due to their possible implementation in spintronic devices. However, the reproducible preparation of highly crystalline chromium(III) halide nanolayers without stacking faults is still an experimental challenge. As chromium trihalides consist of adjacent layers with weak interlayer coupling, the preparation of ultrathin CrX₃ nanosheets directly on substrates via vapor transport proves as an advantageous synthesis technique. It is demonstrated that vapor growth of ultrathin highly crystalline CrX₃ micro- and nanosheets succeeds directly on yttrium stabilized zirconia substrates in a one-step process via chemical vapor transport (CVT) in temperature gradients of 100 K (600°C → 500°C for CrCl₃ and 650°C → 550°C for CrBr₃ or CrI₃) without a need for subsequent delamination. Due to simulation results, optimization of synthesis conditions is realized and phase pure CrX₃ nanosheets with thicknesses ≤25 nm are obtained via short term CVT. The nanosheets morphology, crystallinity, and phase purity are analyzed by several techniques, including microscopy, diffraction, and spectroscopy. Furthermore, a potential subsequent delamination technique is demonstrated to give fast access to CrX₃ monolayers using the example of CrCl₃ .}, language = {en} }