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- ItemGraphene-Like ZnO: A Mini Review(Basel : MDPI, 2016) Ta, Huy Q.; Zhao, Liang; Pohl, Darius; Pang, Jinbo; Trzebicka, Barbara; Rellinghaus, Bernd; Pribat, Didier; Gemming, Thomas; Liu, Zhongfan; Bachmatiuk, Alicja; Rümmeli, Mark H.The isolation of a single layer of graphite, known today as graphene, not only demonstrated amazing new properties but also paved the way for a new class of materials often referred to as two-dimensional (2D) materials. Beyond graphene, other 2D materials include h-BN, transition metal dichalcogenides (TMDs), silicene, and germanene, to name a few. All tend to have exciting physical and chemical properties which appear due to dimensionality effects and modulation of their band structure. A more recent member of the 2D family is graphene-like zinc oxide (g-ZnO) which also holds great promise as a future functional material. This review examines current progress in the synthesis and characterization of g-ZnO. In addition, an overview of works dealing with the properties of g-ZnO both in its pristine form and modified forms (e.g., nano-ribbon, doped material, etc.) is presented. Finally, discussions/studies on the potential applications of g-ZnO are reviewed and discussed.
- ItemTungsten as a chemically-stable electrode material on Ga-containing piezoelectric substrates langasite and catangasite for high-temperature saw devices(Basel : MDPI, 2016) Rane, Gayatri K.; Seifert, Marietta; Menzel, Siegfried; Gemming, Thomas; Eckert, JürgenThin films of tungsten on piezoelectric substrates La3Ga5SiO14 (LGS) and Ca3TaGa3Si2O14 (CTGS) have been investigated as a potential new electrode material for interdigital transducers for surface acoustic wave-based sensor devices operating at high temperatures up to 800 °C under vacuum conditions. Although LGS is considered to be suitable for high-temperature applications, it undergoes chemical and structural transformation upon vacuum annealing due to diffusion of gallium and oxygen. This can alter the device properties depending on the electrode nature, the annealing temperature, and the duration of the application. Our studies present evidence for the chemical stability of W on these substrates against the diffusion of Ga/O from the substrate into the film, even upon annealing up to 800 °C under vacuum conditions using Auger electron spectroscopy and energy-dispersive X-ray spectroscopy, along with local studies using transmission electron microscopy. Additionally, the use of CTGS as a more stable substrate for such applications is indicated.
- ItemEvaluation of surface cleaning procedures for CTGS substrates for SAW technology with XPS(Basel : MDPI, 2017) Brachmann, Erik; Seifert, Marietta; Oswald, Steffen; Menzel, Siegfried B.; Gemming, ThomasA highly efficient and reproducible cleaning procedure of piezoelectric substrates is essential in surface acoustic waves (SAW) technology to fabricate high-quality SAW devices, especially for new applications such SAW sensors wherein new materials for piezoelectric substrates and interdigital transducers are used. Therefore, the development and critical evaluation of cleaning procedures for each material system that is under consideration becomes crucial. Contaminants like particles or the presence of organic/inorganic material on the substrate can dramatically influence and alter the properties of the thin film substrate composite, such as wettability, film adhesion, film texture, and so on. In this article, focus is given to different cleaning processes like SC-1 and SC-2, UV-ozone treatment, as well as cleaning by first-contact polymer Opticlean, which are applied for removal of contaminants from the piezoelectric substrate Ca 3 TaGa 3 Si 2 O 14 . By means of X-ray photoelectron spectroscopy, the presence of the most critical contaminants such as carbon, sodium, and iron removed through different cleaning procedures were studied and significant differences were observed between the outcomes of these procedures. Based on these results, a two-step cleaning process, combining SC-1 at a reduced temperature at 30 ∘ C instead of 80 ∘ C and a subsequent UV-ozone cleaning directly prior to deposition of the metallization, is suggested to achieve the lowest residual contamination level.
- ItemSurface effects and challenges for application of piezoelectric langasite substrates in surface acoustic wave devices caused by high temperature annealing under high vacuum(Basel : MDPI, 2015) Seifert, Marietta; Rane, Gayatri K.; Kirbus, Benjamin; Menzel, Siegfried B.; Gemming, ThomasSubstrate materials that are high-temperature stable are essential for sensor devices which are applied at high temperatures. Although langasite is suggested as such a material, severe O and Ga diffusion into an O-affine deposited film was observed during annealing at high temperatures under vacuum conditions, leading to a damage of the metallization as well as a change of the properties of the substrate and finally to a failure of the device. Therefore, annealing of bare LGS (La 3 Ga 5 SiO 14 ) substrates at 800 ∘ C under high vacuum conditions is performed to analyze whether this pretreatment improves the suitability and stability of this material for high temperature applications in vacuum. To reveal the influence of the pretreatment on the subsequently deposited metallization, RuAl thin films are used as they are known to oxidize on LGS at high temperatures. A local study of the pretreated and metallized substrates using transmission electron microscopy reveals strong modification of the substrate surface. Micro cracks are visible. The composition of the substrate is strongly altered at those regions. Severe challenges for the application of LGS substrates under high-temperature vacuum conditions arise from these substrate damages, revealing that the pretreatment does not improve the applicability.
- ItemCapability study of Ti, Cr, W, Ta and Pt as seed layers for electrodeposited platinum films on γ-Al2O3 for high temperature and harsh environment applications(Basel : MDPI, 2017) Seifert, Marietta; Brachmann, Erik; Rane, Gayatri K.; Menzel, Siegfried B.; Gemming, ThomasHigh temperature surface acoustic wave sensors based on radio frequency identification technology require adequate antennas of high efficiency and thermal stability for the signal transmission. Platinum is well known and frequently used as a material of choice for high temperature and harsh environment applications because of the high melting point and its chemical stability. Therefore, one way to realize high temperature stable antennas is the combination of a Pt metallization on an Al 2 O 3 substrate. As a cost-effective technique, the Pt film is deposited via electrochemical deposition. For this growth procedure, a pre-deposited metallization on the Al 2 O 3 layer is required. This paper analyzes the influence of various seed layers (Ta, Ti, W, Cr, Pt) on the morphology, stability and electrical properties of the electrochemically-grown Pt thick film after heat treatments up to 1000 ∘ C in air. We find an oxidation of all adhesion layers except for Pt, for which the best electrical properties were measured. Although significant areas of the films delaminate from the substrate, individual anchor structures retain a stable connection between the Pt layer and the rough Al 2 O 3 substrate.
- ItemThe Influence of the Composition of Ru100−xAlx (x = 50, 55, 60, 67) Thin Films on Their Thermal Stability(Basel : MDPI, 2017-3-10) Seifert, Marietta; Rane, Gayatri K.; Oswald, Steffen; Menzel, Siegfried B.; Gemming, ThomasRuAl thin films possess a high potential as a high temperature stable metallization for surface acoustic wave devices. During the annealing process of the Ru-Al films, Al2O3 is formed at the surface of the films even under high vacuum conditions, so that the composition of a deposited Ru50Al50 film is shifted to a Ru-rich alloy. To compensate for this effect, the Al content is systematically increased during the deposition of the Ru-Al films. Three Al-rich alloys—Ru45Al55, Ru40Al60 and Ru33Al67—were analyzed concerning their behavior after high temperature treatment under high vacuum and air conditions in comparison to the initial Ru50Al50 sample. Although the films’ cross sections show a more homogeneous structure in the case of the Al-rich films, the RuAl phase formation is reduced with increasing Al content.
- ItemProduction of porous β-Type Ti–40Nb alloy for biomedical applications: Comparison of selective laser melting and hot pressing(Basel : MDPI, 2013) Zhuravleva, Ksenia; Bönisch, Matthias; Prashanth, Konda Gokuldoss; Hempel, Ute; Helth, Arne; Gemming, Thomas; Calin, Mariana; Scudino, Sergio; Schultz, Ludwig; Eckert, Jürgen; Gebert, AnnettWe used selective laser melting (SLM) and hot pressing of mechanically-alloyed β-type Ti–40Nb powder to fabricate macroporous bulk specimens (solid cylinders). The total porosity, compressive strength, and compressive elastic modulus of the SLM-fabricated material were determined as 17% ± 1%, 968 ± 8 MPa, and 33 ± 2 GPa, respectively. The alloy’s elastic modulus is comparable to that of healthy cancellous bone. The comparable results for the hot-pressed material were 3% ± 2%, 1400 ± 19 MPa, and 77 ± 3 GPa. This difference in mechanical properties results from different porosity and phase composition of the two alloys. Both SLM-fabricated and hot-pressed cylinders demonstrated good in vitro biocompatibility. The presented results suggest that the SLM-fabricated alloy may be preferable to the hot-pressed alloy for biomedical applications, such as the manufacture of load-bearing metallic components for total joint replacements.
- ItemColloidal PbS nanoplatelets synthesized via cation exchange for electronic applications(Cambridge : RSC Publ., 2019) Sonntag, Luisa; Shamraienko, Volodymyr; Fan, Xuelin; Samadi Khoshkhoo, Mahdi; Kneppe, David; Koitzsch, Andreas; Gemming, Thomas; Hiekel, Karl; Leo, Karl; Lesnyak, Vladimir; Eychmüller, AlexanderIn this work, we present a new synthetic approach to colloidal PbS nanoplatelets (NPLs) utilizing a cation exchange (CE) strategy starting from CuS NPLs synthesized via the hot-injection method. Whereas the thickness of the resulting CuS NPLs was fixed at approx. 5 nm, the lateral size could be tuned by varying the reaction conditions, such as time from 6 to 16 h, the reaction temperature (120 °C, 140 °C), and the amount of copper precursor. In a second step, Cu+ cations were replaced with Pb2+ ions within the crystal lattice via CE. While the shape and the size of parental CuS platelets were preserved, the crystal structure was rearranged from hexagonal covellite to PbS galena, accompanied by the fragmentation of the monocrystalline phase into polycrystalline one. Afterwards a halide mediated ligand exchange (LE) was carried out in order to remove insulating oleic acid residues from the PbS NPL surface and to form stable dispersions in polar organic solvents enabling thin-film fabrication. Both CE and LE processes were monitored by several characterization techniques. Furthermore, we measured the electrical conductivity of the resulting PbS NPL-based films before and after LE and compared the processing in ambient to inert atmosphere. Finally, we fabricated field-effect transistors with an on/off ratio of up to 60 and linear charge carrier mobility for holes of 0.02 cm2 V−1 s−1.
- ItemA size dependent evaluation of the cytotoxicity and uptake of nanographene oxide(London [u.a.] : RSC, 2015) Mendes, Rafael Gregorio; Koch, Britta; Bachmatiuk, Alicja; Ma, Xing; Sanchez, Samuel; Damm, Christine; Schmidt, Oliver G.; Gemming, Thomas; Eckert, Jürgen; Rümmeli, Mark H.Graphene oxide (GO) has attracted great interest due to its extraordinary potential for biomedical application. Although it is clear that the naturally occurring morphology of biological structures is crucial to their precise interactions and correct functioning, the geometrical aspects of nanoparticles are often ignored in the design of nanoparticles for biological applications. A few in vitro and in vivo studies have evaluated the cytotoxicity and biodistribution of GO, however very little is known about the influence of flake size and cytotoxicity. Herein, we aim at presenting an initial cytotoxicity evaluation of different nano-sized GO flakes for two different cell lines (HeLa (Kyoto) and macrophage (J7742)) when they are exposed to samples containing different sized nanographene oxide (NGO) flakes (mean diameter of 89 and 277 nm). The obtained data suggests that the larger NGO flakes reduce cell viability as compared to smaller flakes. In addition, the viability reduction correlates with the time and the concentration of the NGO nanoparticles to which the cells are exposed. Uptake studies were also conducted and the data suggests that both cell lines internalize the GO nanoparticles during the incubation periods studied.
- ItemIn Situ Room Temperature Electron-Beam Driven Graphene Growth from Hydrocarbon Contamination in a Transmission Electron Microscope(Basel : MDPI, 2018-5-26) Rummeli, Mark H.; Pan, Yumo; Zhao, Liang; Gao, Jing; Ta, Huy Q.; Martinez, Ignacio G.; Mendes, Rafael G.; Gemming, Thomas; Fu, Lei; Bachmatiuk, Alicja; Liu, ZhongfanThe excitement of graphene (as well as 2D materials in general) has generated numerous procedures for the fabrication of graphene. Here we present a mini-review on a rather less known, but attractive, in situ means to fabricate graphene inside a transmission electron microscope (TEM). This is achieved in a conventional TEM (viz. no sophisticated specimen holders or microscopes are required) and takes advantage of inherent hydrocarbon contamination as a carbon source. Both catalyst free and single atom catalyst approaches are reviewed. An advantage of this technique is that not only can the growth process be imaged in situ, but this can also be achieved with atomic resolution. Moreover, in the future, one can anticipate such approaches enabling the growth of nano-materials with atomic precision.